Brit's mom is already corrupted

She's an evil bitch according to Neon (and also garnered from Brit herself when she discusses her a little). Now,
punishing her... that's a whole other story
Lots of people like incest play, but that doesn't mean they want actual incest. That's Neon's intention w/ Ashe if I had to bet. Plus, her play so far is just brother/sister, which is actually not all that weird for someone who doesn't even have a brother imo. Well, I guess she has called MC daddy, but I feel like that's so mainstream at this point and most chicks aren't thinking of their actual father, it's more like a dom/sub "you're taking good care of me" (sexually, financially, etc.) kind of thing. It's like calling your SO 'baby'; you're not thinking of them as an actual child, just someone you want to take care of.
Bonnie's for sure in that same boat, if she even has an incest play kink herself (doubt it honestly). I think a mom-daughter threesome for them (and Madison & Gabby too) is just too ridiculous for this game tbh, I can't work it out in my head as being anything other than farcical, even though it's really hot to think about.
Brit's mom might work for that, though, depending on how despicable and abusive she truly is. We know very little, but considering how narcissistic she sounds, I could see her lowering herself to letting MC fuck her alongside her daughter if it meant lots of money for her, even if she had zero sexual interest in it.