Genre Family Sizes

We all know fantasy families are too small because it's a pain to come up with names for a bunch of siblings, cousins, etc., but surely science fiction should be dominated by the only child or single sibling absent cloning, right? Look at contemporary fertility. But wait! If you're writing about future people, they must have had future parents and so on down the line, which is to say, they'll belong to cultures which keep those birthrates up. Again, cloning excluded.