At this point...With DS's conception of what makes Incest happen and Anne (and everyone else's to be honest) predilection for being CONTROLLED or FORCED into doing things...
I have to wonder why he just hasn't put a gun into the Online Store So Max can just buy one and fuck everyone at gunpoint...Game Over! LOL
I dunno how I skipped this comment, but I lol'd.
One thing I noticed in DS' characters is that they have anything BUT consistent personalities.
For example:
Lisa plays the innocent little sister, so far so good, then she starts getting dirtier and dirtier with her brother untill they reach full fledged anal sex anywhere, then she goes back to "I want a boyfriend, someone who I can trust to give my first time" and starts going to dates with max just to shortly after that go back to "Meh, I don't mind you fucking other girls, as long as I can watch, maybe, just maaaaaybe I'll join".
Ann starts as the strict mother, that uses physical punishments (aka spanking) too "keep her children in line", but then she turns out to be a submissive woman who's not right in the head, then she turns bissexual with her own sister and then becomes porn actress, all that while still hypocritically using her strict mother persona.
Alice starts as the stereotypical elder sister, authoritative, loathes his younger brother but then turns out to be a submissive bisexual who would even let her brother fuck her if it pleases her dom, then she suddenly turns into a dom herself.
Yep, fickle as fickle can be.
My read on DS's conception on what makes ladies do crazy things like incest is Max's "big gun" resting between his legs -- that's why he's the "Big Brother" after all (according to the title of the game).
On a more serious note, you're right, and this sort of thing is permissible for Lisa, Max and Alice because they're kids -- they haven't figured themselves out.
The challenging part is to figure out Ann's neurosis -- I mean, at one point, she calls Max her rock -- I mean, lol, wut?! Lisa is more spot on in saying Max is a pervy moron because the activities I as a player directing Max are pervy, boyish and moronic -- peep lots, put cameras all over the house, lay spiders in Alice's bedroom, etc.
One way I would straighten out the story to make it more thematic:
Max starts out as a peeper, then grows into being a private voyeur spying on his sisters through the cameras he installs all over the house, and then graduates to being forced to share those cameras on the Internet to generate money due to being at war with Eric so that he can become the provider of the house -- a big brother in more ways than the only "big brother" feature he has right now: that monster that rests between his legs.
I can grow the story from there by trying to involve Alice on my money-making site by generating revenue on her room through that blog opportunity -- just make that opportunity earlier so that it's focused on generating money to make Max the Alpha male around instead of Eric. Then reward the player with pervy scenes with Mom when Max transcends Eric's influence/domination with Mom because Mom's focus is providing for the family, one of the reasons why she hooked up with Eric in the first place (the other being that she's broken because of the trauma she went through while growing up).
TLDR: the framework for getting this done is already in the game -- the opportunity list are the plots DS incorporated into the game, and tbh, that list is long in my opinion as a writer. One reason I like the game very much is because those plots are detailed and interesting, and I think it's going to take quite some time to tie them all up into a coherent whole, story-wise (as in, getting the story details of the plots correct, coherent and believable, is what I meant).