Again, that's your opinion. I don't see her breaking any rules at all. I see her as just being drunk and sleazy.
She might have had a lot of sex with other guys, but what does that proof ? Everyone in this game is sexually liberated and as Snugglepuf says it doesn't take much imagination picturing her having sex with her father, but one thing is clearly forbidden:
vaginal sex between family members. Easy to figure out why that would be, doesn't it ?
Even more, even if it was not taboo, if she gets pregnant, she would have to stop drinking. That alone could be reason enough why she doesn't want to risk that