My only concern is that a incestuous relationship faces serious social stigmas if it becomes revealed. It's a huge strain on a relationship having to hide it from everyone, always with the nagging feeling that you are doing something wrong.
I don't care what two consenting adults get up to, it's not my concern so long as no one is being forced into anything. Despite having a close relationship with my little sister (both adults now) it's always been pretty normal, I never thought about her in that way. Even though incest doujins are hot, it rarely works out like that.
But whatever, not something I feel too strongly about either way.