usually easiest way is to get them into a trance (using serum) or making them orgasm a lot. once they're in a trance, you can usually break their taboos (like weakening their hate for incest etc)How to break the taboo?
if it's the positions unlocked that your'e referring to (like gaining access to oral, penetration, anal etc) i think you have to progress their story or sex them up, make them cum/yourself cum like 5x in the previous position?