It doesn't even matter if he likes incest or not. An incest-lover could easily also fall into the trap of calling his incest-partner his "girlfriend" and having a large part of the story then not make sense.
The game shouldn't give a free choice of roles on a character that actually has a very specific role in-story; that's all. Adding a check to see if that character has a "family member" title and doing the incest stuff if so (making the incest stuff optional, and easy to miss in its entirety) would make far more sense for the censored version.