Absolutly agree on that. It should start with Kira finding out by chance and peeking on Max and Alice on what they're doing in the bathroom.
Kira's reaction should be of surprised at first, even shocked. Not something like "oh, it was expected" or some bullshit like that. No, it isn't expected siblings to fool around even if drunk, come on.
Later then things could lead to those FFM action.
As for the sex ed stuff, i really hope it isn't included. In all iterations, remakes and fangames these scenes always felt forced, stupid and a cheap way to access Ann in a sexual way.
I do like your idea of the modeling though. That should be the way.
Yeah to me the modeling would be a good first start. Personally I think a good next step would be to make Ann kind of a voyeur and catch Kira and Max being increasingly naughty while watching TV in the living room. At first they're just watching TV with their clothes on. It might be something R rated and Kira isn't exactly wearing much, but nothing too scandalous. But before long she catches Kira giving Max kissing lessons which she probably would have some kind of talk with Kira and Max about what's up with that. Then she starts catching them watching full on porn, masturbating in front of each other, going down on each other, and eventually fucking.
At first she should be pissed off at both of them and interrupt them. Eventually she would decide not to interrupt and just watch, preferably while masturbating herself. She now has an excuse to be aroused by Max since he's being naughty with someone other than herself or either of her daughters. Kira is old enough not to need her protection and is as much to blame if not moreso than Max.
That's the point where I think escalating the modeling stuff would make sense. She's already used to being naked or close to it while Max takes her picture. She's also used to seeing Max fucking Kira (or at least doing a lot more than you're supposed to do with an Aunt).
I don't think it's realistic to have her catch Max doing things with either Alice or Lisa and be even remotely ok with it (at least not until late game). If she's going to give Sex Ed lessons to Lisa, IMO it shouldn't happen until at a bare minimum she has caught Max Fucking Kira and decided not to freak out about that anymore. I would argue that finding out about Alice and getting to where she accepts that should also be required for her to move on to Lisa. Lisa should be the one she wants to protect the most.
I think Lisa's path should focus more on both her and Olivia having increasingly naughty "sleepovers" when Olivia comes over. Personally, I think that means Lisa goes at least part way down the lesbo path with Olivia and watches Olivia go all the way with Max before really going all that far with Max herself.