I got to thinking -- not that it's going to be relevant, of course -- about what happens if one of the older siblings gets killed while the parents are still having kids. And so far I don't have a very good story-related way to say that it should work one way or another, so here I am asking you:
How should this go?
The only thing that matters is birth order, not anything since. If five of your older siblings have been killed by trolls, you still get the powers of #6.
The chemistry model, filling the available valences: if your oldest sib has been killed by trolls before your birth and there are four more between you, you get the powers of #1, not #6.
Everybody moves up when someone dies: you are your parents' oldest *living* child, second *living* child, etc.
Something else I will specify in comments.
If your answer is, "Don't do it that way at all," please just don't answer.
I'm also very charmed by the elephant selkies timprov and I have been discussing. "Because selkies are all 'oooh, I have soft brown eyes,'" I said, "and then, hrrrronnnnnnnk!" There will also be walkies. Not the kind I'm taking Ista on in a minute, either. The kind with tusks. But not in this thing. Not until the next thing, because there's not room in this thing for half of what's in it, much less more stuff, and I know more stuff will sneak in accidentally. So we will plan the elephant selkies ---hrrrrronnnk! -- for later.
Now I'm going around the house going hrrrronnnnk!
Because I am delicate and demure, is why.