Well, there's clearly a (lucrative) market for it if this SP is anything to go by...External Link/Members Only
And given the huge amount of incest and pseudo-incest porn there is on the web, these sorts of things are clearly a common fantasy. The number of escorts on AW offering taboo roleplay, sometimes with more explicit descriptions of what's on offer, suggests that lots of punters want to take this into the roleplay arena.
Consensual non-consensual (CNC) roleplay is a similar area where (I hope) punters are just playing out fantasies and wouldn't really want to coerce women. Again, escorts offer this, and sometimes more exlicitly, although fewer than the taboo providers.
Then there is the question of where you stand on animal roleplay, for which there is a big market, both sexual and non-sexual (just dressing up and playing).
I have no answers, beyond the belief that if it doesn't hurt anyone, then each to their own. However, it might be worth considering, even more so than in "regular" punting how your real family might feel about these roleplays.