I mostly agree with the "No one is forcing you guys" comment, but I think this is a very valid point too. I like it when major thresholds of corruption (especially incest corruption) are justified, when you read the dialogue or explanations of the character's behavior and think "that kinda makes sense, I can suspend my disbelief for that." They try to do something like that here for the mom and sister with the taboo break tasks, but those feel more like arbitrary fetch quests. "Clean up the house and I'll give you a handjob" is something a mom would say after she's already corrupted and become okay with the idea of being sexual with her son. It's not something that credibly explains why she would cross that threshold.
You know, for this game in particular, I feel like what would make the most sense is for the incest taboo breaks to unlock by giving them particular serums. For instance, if you want the naked or handjob taboo break, you might be tasked to feed them a serum that does +10 Obedience, +10 Sluttiness, and +10 Arousal. Basic tier 1 stuff. But then for vaginal, you'd have to concoct some very powerful serum that requires either getting to the highest tier of research, or having unlocked Nora's research serum effects and some really good nanobots. Or alternately, you could buy some super-secret illegal serum trait that covers all the requirements, but costs a shit-ton of money to obtain. Either way, you actually have to play the game and be getting shit done on the business side of things to effectively corrupt your family.
Of course, there should also be a pure obedience route where you can just order them to do it, but it should be a bit more difficult and risky. Like, being commanded to do sexual stuff they're not comfortable with should make them more unhappy, and then maybe they'll just completely refuse to engage with you at all (or not come home for a long time) if they're too unhappy. So you would need to figure out what you can command them to do that they secretly want to do, so it makes them more happy.
God, I wish I had programming skills.