I just said the same thing. You can't expect Liza to casually date her little brother under no circumstances so at least the start of their relationship will have to be something that forces her to put up with him even if he is nice to her and showers her with money and care immediately afterwards.
What do you mean by corruption? I can see how you can make a path where Bobby is exposing a 12 year old cousin to sex and gradually getting her curious and horny, but how the hell do you expect Bobby to corrupt his older sister who is opposed to incest, is not sexually attracted to her brother and is already in a relationship with another guy? It makes no sense.
Não é a mesma coisa.
# Corruption is exactly that: make something imoral as incest feel/look normal, acceptable or at least tolerable when secret...
I never said it would be easy (in a realistic sort of way) but it's doable: fake adoption certificate, fooling around/harmless horseplay gone wrong, gradual bribing, no moral values like a stripper/hooker/escort/sugar baby... (one must make use of imagination) Tricked but still willing.
Another thing is Abuse: alcohol, pills, sleep, drugs,... All fine by me (Not wiling, but barely aware so...)
But Breaking her spirit is playing on a different field. It's psychological terror. This can only end badly (arrested, committed, death/injury, broken home,...). Some people may not like this in a game. I don't mind as long as it makes sense.
(Blackmail is grey area but being blackmailed and being broken are still very different - depth)