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The Importance of Siblings
So, as part of Hugos, I have read a lot more different original-worlds in a very short time than I have for awhile, so here is a worldbuilding rant aimed at Most Of The People Who Wrote Secondary-World Stories I Read:
If you are writing a world with a high death rate among young people - whether it's due to disease, war, accidents, space vampires, cage-fighting, spectacularly failing the magical initiation, your paper-and-balsa-wood wings failing at a terrible moment, being dragged away by Mysterious Masked Figures, whatever - your characters should not all be only children.
Even in a world with a low death rate among the young, you need a birthrate of slightly-more-than-2-per-uterus-having-person for a stable population of humans. With a high death rate, you need a birthrate substantially higher than 2-per. (Also, dear stories I read for the Hugos, if your base family unit is, say, triads of women, your average family size does in fact need to be six kids. If it's two men and one women, then a, what are you doing to all your girlchildren to get that gender ratio, and b, your average family size should be at minimum three kids. Or possibly two boys, a girl, and a baby girl you exposed at birth.)
If you're picking off a significant number of your young people before they breed, and none of them have siblings to carry on the line, your society is not going to last very long. With a high-death-rate society, two kids should be a small family. And they should have a lot of first cousins. A lot of them will be dead at a young age, but hey, you're the one who decided to build a high-mortality society.
(You also need your characters to have lots of siblings and cousins if your society is actively expanding into new land; if your society has a lot of outward migration; or if rampant population growth is a plot point of your story. Also, if a lot of your people of childbearing biology don't ever have children, the ones who do have kids need to have more kids to cover for the childless ones, so people will still have siblings even if they don't have kids.)
You can get away with fewer siblings if your culture is getting a lot of inward migration - if it's a city that's growing due to migration from the countryside, for example; or if it's an elite group of warrior-mages who bring in talented people from the outside population. But in that case, your society needs to show evidence that it's structured to take in a lot of people raised outside its culture, and your people from the outside population need to have siblings, since their culture is supporting both their own population growth and the elite group's.
Other than that, if all your characters are only children of only children, a rapidly shrinking population had better be a plot point.
(obviously if you go all-out to, idk, growing your characters in vats from gene-banks, or if they are left on doorsteps by storks, or if you have 0 mortality, all of this will apply very differently, but it'll still apply somehow.)
I get the reasons why authors write so many characters from small families. Most people getting published in English these days are from societies where small families are the norm, and especially in YA, are writing for an audience who are presumed to be from smaller families. (Although this is less true than you might think: a lot of modern low-birth-rate populations are doing the 'a lot of women don't have kids, the ones who do have large families' thing instead of 'everyone's an only child'.)
And it also greatly simplifies your storytelling not to have all those extraneous relations hanging around. And having your main character being the Last of Their Line is built-in Drama. So make your main character in a high-mortality society an only child with no cousins if you must. But they had better be an exceptional case, and your secondary characters better have siblings and cousins and aunts and uncles. And there had better be in-story justifications for why they are an exception.
And 'one of their parents died or disappeared when they were a baby' is not a good enough justification; in a high-mortality society there will be strong incentives for a single parent of reproductive age to remarry and give them half-siblings.
Bujold's Barrayar is a good example of how to do this well: Barrayar, at the time of Miles' birth, is a fairly high-mortality society between war and poor health care, and it's also actively settling a new continent and about to colonize a new planet and expanding militarily, and yet her three main characters in that generation, Miles, Ivan, and Gregor, are all only children with no first cousins. But there are story-relevant reasons for this: Ivan and Miles' fathers did have siblings, but they were all dead before marriage; Miles' parents want more kids and are under family pressure to have more, but know it would screw things up for Miles and Gregor if they did, so they resist the pressure; Gregor's mom actually was going to remarry and have more kids but got murdered first; Ivan's mother is under social pressure to remarry but resists it and is considered exceptional. Plus, Ivan's mother and Miles' mother do both start new relationships at an age when they could still have kids, though I haven't read the newest book so don't spoil me on whether they have yet in canon. And Miles actually does have maternal cousins, they just aren't a major part of his life for political reasons. And pretty much all of the other Barrayaran characters whose families we know about do have multiple siblings, and people constantly remark on the fact that it's weird that Miles and Ivan and Gregor don't. So Bujold gets away with it.
Bad example: Naruto. Okay I'm only about 1/4 of the way in, so maybe it gets better as Kishimoto expands the world, but so far we have a society where most people assume they will be violently dead by age thirty, and where there's a cultural emphasis on large lineal clans carrying forward certain bloodlines, and yet nearly all the main characters are only children, and the ones who aren't have only a single sibling. There is only one family so far with as many as three siblings (and the third sibling is treated as an unusual extra), and nobody has more than one aunt or uncle, either. And most of the characters who are in their twenties have no kids. Main character Naruto gets away with having no siblings because his parents both died the day he was born. But the fact that he has no aunts, uncles, or first cousins, and nobody finds this unusual, and the fact that nearly everyone he knows, even the ones who are supposed to part of large powerful sprawling clans, have at most one sibling and generally none, is bothering me more and more the longer I read.
So, conclusion: If you are killing a lot of characters, make sure most of them have siblings, and at least some of them have lots of siblings. You can have most of the siblings be dead or in another country if you need them out of the way, but they should at least exist.
If you are writing a world with a high death rate among young people - whether it's due to disease, war, accidents, space vampires, cage-fighting, spectacularly failing the magical initiation, your paper-and-balsa-wood wings failing at a terrible moment, being dragged away by Mysterious Masked Figures, whatever - your characters should not all be only children.
Even in a world with a low death rate among the young, you need a birthrate of slightly-more-than-2-per-uterus-having-person for a stable population of humans. With a high death rate, you need a birthrate substantially higher than 2-per. (Also, dear stories I read for the Hugos, if your base family unit is, say, triads of women, your average family size does in fact need to be six kids. If it's two men and one women, then a, what are you doing to all your girlchildren to get that gender ratio, and b, your average family size should be at minimum three kids. Or possibly two boys, a girl, and a baby girl you exposed at birth.)
If you're picking off a significant number of your young people before they breed, and none of them have siblings to carry on the line, your society is not going to last very long. With a high-death-rate society, two kids should be a small family. And they should have a lot of first cousins. A lot of them will be dead at a young age, but hey, you're the one who decided to build a high-mortality society.
(You also need your characters to have lots of siblings and cousins if your society is actively expanding into new land; if your society has a lot of outward migration; or if rampant population growth is a plot point of your story. Also, if a lot of your people of childbearing biology don't ever have children, the ones who do have kids need to have more kids to cover for the childless ones, so people will still have siblings even if they don't have kids.)
You can get away with fewer siblings if your culture is getting a lot of inward migration - if it's a city that's growing due to migration from the countryside, for example; or if it's an elite group of warrior-mages who bring in talented people from the outside population. But in that case, your society needs to show evidence that it's structured to take in a lot of people raised outside its culture, and your people from the outside population need to have siblings, since their culture is supporting both their own population growth and the elite group's.
Other than that, if all your characters are only children of only children, a rapidly shrinking population had better be a plot point.
(obviously if you go all-out to, idk, growing your characters in vats from gene-banks, or if they are left on doorsteps by storks, or if you have 0 mortality, all of this will apply very differently, but it'll still apply somehow.)
I get the reasons why authors write so many characters from small families. Most people getting published in English these days are from societies where small families are the norm, and especially in YA, are writing for an audience who are presumed to be from smaller families. (Although this is less true than you might think: a lot of modern low-birth-rate populations are doing the 'a lot of women don't have kids, the ones who do have large families' thing instead of 'everyone's an only child'.)
And it also greatly simplifies your storytelling not to have all those extraneous relations hanging around. And having your main character being the Last of Their Line is built-in Drama. So make your main character in a high-mortality society an only child with no cousins if you must. But they had better be an exceptional case, and your secondary characters better have siblings and cousins and aunts and uncles. And there had better be in-story justifications for why they are an exception.
And 'one of their parents died or disappeared when they were a baby' is not a good enough justification; in a high-mortality society there will be strong incentives for a single parent of reproductive age to remarry and give them half-siblings.
Bujold's Barrayar is a good example of how to do this well: Barrayar, at the time of Miles' birth, is a fairly high-mortality society between war and poor health care, and it's also actively settling a new continent and about to colonize a new planet and expanding militarily, and yet her three main characters in that generation, Miles, Ivan, and Gregor, are all only children with no first cousins. But there are story-relevant reasons for this: Ivan and Miles' fathers did have siblings, but they were all dead before marriage; Miles' parents want more kids and are under family pressure to have more, but know it would screw things up for Miles and Gregor if they did, so they resist the pressure; Gregor's mom actually was going to remarry and have more kids but got murdered first; Ivan's mother is under social pressure to remarry but resists it and is considered exceptional. Plus, Ivan's mother and Miles' mother do both start new relationships at an age when they could still have kids, though I haven't read the newest book so don't spoil me on whether they have yet in canon. And Miles actually does have maternal cousins, they just aren't a major part of his life for political reasons. And pretty much all of the other Barrayaran characters whose families we know about do have multiple siblings, and people constantly remark on the fact that it's weird that Miles and Ivan and Gregor don't. So Bujold gets away with it.
Bad example: Naruto. Okay I'm only about 1/4 of the way in, so maybe it gets better as Kishimoto expands the world, but so far we have a society where most people assume they will be violently dead by age thirty, and where there's a cultural emphasis on large lineal clans carrying forward certain bloodlines, and yet nearly all the main characters are only children, and the ones who aren't have only a single sibling. There is only one family so far with as many as three siblings (and the third sibling is treated as an unusual extra), and nobody has more than one aunt or uncle, either. And most of the characters who are in their twenties have no kids. Main character Naruto gets away with having no siblings because his parents both died the day he was born. But the fact that he has no aunts, uncles, or first cousins, and nobody finds this unusual, and the fact that nearly everyone he knows, even the ones who are supposed to part of large powerful sprawling clans, have at most one sibling and generally none, is bothering me more and more the longer I read.
So, conclusion: If you are killing a lot of characters, make sure most of them have siblings, and at least some of them have lots of siblings. You can have most of the siblings be dead or in another country if you need them out of the way, but they should at least exist.