I don’t think there’s any indication that Kasimir’s reason for wanting to bring his sister back is for breeding purposes. He rather explicitly wants to bring her back out of love and guilt. But regardless of his intent, the implication of her being the only living dusk elf capable of bearing children is that either the other dusk elves use her for that purpose, or they continue going slowly extinct (ignoring that half-elves are a thing, as the book seems to forget that’s an option). That’s going to be over the line for some players, and I think it’s worthwhile to consider alternative possibilities.