I always thought it odd that Mom would believe fucking Eric more than her own son.
Yes, she misinterprets it as child resentment, but unless his jizz has mind-raping properties just threatening to boot him out of the house "because Eric says to" has always struck me as just unbelievable, especially instead of sitting the two of them down together for a talk - which is what intelligent humans would do.
I know he's a manipulator (who frankly seems threatened by the MC's presence... talk about pathetic) but I can't get over how fucking SATISFYING it was to have him fucked out and probably killed by the people he owes money to.
You're forgetting something important... Every single person in the family is fucked up, way more than is remotely believable.
Examining the characters and their personalities takes suspension of disbelief to a whole new level of impossible, unless you remember that they're all freaks.
This is a porn game, with a loose plot all centred around Max having sex with almost everyone who's not Eric. Accept that, and all the story/character problems mean nothing.
Literally none of them would do the things they do, or wind up doing, if they weren't interested in doing them.
Alice already wants to fuck her brother, and is easily the first one to suck his cock.
Lisa takes practically no convincing, and turns out to love being sexually submissive like her mother, and particularly for her brother, and gladly does the same with her older sister and brother at the same time, under the pretence of "wanting to be closer with her sister".
Kira's just Kira, and she's been the way she is from a young age, and is more than willing to take her nephews virginity by any means - Takes advantage of him, lures him into taking enticing and then blatant pictures of her, and either allows him to have sex with her, or she has sex with him after tying him to his mother's bed.
Then she wants to have her entire family involved in making porn. Together. With each other.
Ann's damaged goods because of what happened when she was young (the whole being held prisoner thing), and no doubt has been thinking about Max, her son, in a sexual way for some time. For almost all of the time that Eric's still in the game, she's basically trying to be a normal parent, but that goes out the window once she's allowing her own son to massage her practically naked body, and letting him watch her get dressed.
The crowning achievement of her poor attempt to appear normal, is masturbating her own son after a massage or in the shower.
Hell, just the fact that she can be persuaded into almost anything through sex is pretty ridiculous in its own right.
Best of all, by the end she's clinging to the illusion of Lisa not being involved, but is either sucking her son's cock in front of her, or watching her suck it instead, and happily having sex with her son alongside her eldest daughter too.
Everyone else who's not related to the family, aside from Eric, is best described as "Max-sexual"...