Can't be hentai and not twincest? You're not going to find any.

There are some series where twincest is a serious plot element, or not just a gag. That's about all you'll find.

There is the off-chance a series will have what you're looking for, but it will be throwaway background info used as a shock, and have no relevance to the plot or actual character development (could be substituted for some other vile shock factor twist).