Well obviously, after so much time spent corrupting Ann, she will allow it. In fact, she wants it too, but needs an excuse to do so. Know Yandere Simulator? Yandere dev had an issue with his game taking 90 seconds to compile and load every time he wanted to test new stuff. As a result, he'd easily find himself the excuse "I'll just watch a youtube video while it's loading", excuse to avoid doing something he didn't like anymore (luckily it's changed since). This is the same psychological process, but for "banned stuff that you really want to try". The eros from doing something forbidden is far greater than the eros of "we'll bang okay" - which explains many plotlines requiring persuasion then no longer.
For Lisa and the book, it doubles as fapping material. When Max gives her the playboy magazine, she's like "uh... I wanted fap material genius". Also, before the lessons start, it's strongly hinted she knows perfectly what Max has in mind. She even warns him against stupid requests. She may be inexperienced and playing her "innocent young sister" card, she's far from stupid and voluntarily goes along Max's plans, but tries to do so while keeping control over the situation if at any point she decides she wants to stop.
Lastly regarding Ann, I agree with you. Partly or not, dunno but when you say you don't really care when it happens so long as it makes sense story-wise, I'm 100% with you. Why partly, because this happening due to the other characters pulling strings may not be the only way this will be achievable in the end. If it is the only way in the final product, then I'd join you and say "yeah no, this forces the player unto a roadtrail in a sandbox game". And the trauma or backstory with the father, don't know enough yet to say anything so it's in the air for now. Hopefully it'll bring good (as in interesting/entertaining) developments.
Edit: And I have to edit because DeUglyOne's comment is... very touching. It's a shame for the old couple, accursed drunk driver. But both couples are pretty much why I don't knee jerk to incest like I used to. I may not have any interest in any sibling of mine, knowing that some people would go against "common sense" and general opinion just because they love each other that much, I find it truly heartwarming. They have my best wishes and I hope they stay happy together!