Do shrimp really have sex with their siblings before being served raw in sushi?
That's not really true in any meaningful way, and has nothing to do with sushi. It sounds like the author was just trying to make it sound scandalous to attract attention.

It's a really, really sensationalized version of something true about at least some species of shrimp. They're essentially all born male, so before they can reproduce, naturally, some of them have to turn into females. The bit about siblings is essentially made up; sure, each shrimp has a lot of offspring, so some probably do randomly reproduce with a "sibling", but it's not like they're seeking it out. The younger part is true in a sense, though, since they turn from male to female but not back, so naturally the males are younger than the females. See for example this page.

I don't think those species are the only ones used in sushi, though, so all in all, the statement is essentially false. Not all shrimp change sex, in the species which do that, not only older ones (which have changed sex) are caught, and not all of them mate with siblings. I'm sure someone has eaten sushi made with a female member of one of those species which did happen to have mated with a sibling, but that's nothing to write a book about.