Everything up to this point has been from Max's point of view though, unless I'm forgetting something. Breaking from that would be kind of weird this late in the game. I'm not saying that it's not possible, because I'm not ruling out anything. But it would be an odd departure.
I think Ann's mental stability is debatable. She had tremendous psychological trauma when she was younger. Then look at her complete devotion to Eric when he was around despite Max offering clear evidence of what he was. There's her lounging around the house in nothing but a towel most of the day, which doesn't seem particularly healthy. Or the fact that she continues to fall for Max's story about carpal tunnel syndrome and allows her kids to be sexually active with each other despite harboring a clear bias against incest. Unless she's secretly okay with all of it, there's something seriously wrong with her. But if she is okay with it, she should have given in herself long ago.
But I agree on the desperation thing. If being naked with Max and giving him blowjobs or receiving sensual massages from him doesn't make her want to have sex with him after months of no sex at all, I don't see why filming a lesbian bdsm scene away from home is going to translate into heterosexual incest at home. And what's the fun or feeling of accomplishment in having sex with a woman who was aroused by someone else? But this game stopped making sense a long time ago.
