Are u implying an already existing degeneracy in the family????
Nergal forbid...
But I will say that I am not a fan (personally) of M+S or B+S incest. This goes for all sibling, parental incest tbh. But there have been some exception depending on the game (I can live with F+D or B+S to an extent). In this game the incest is tolerable for me because they are monster daughters (there not human so they dont play by the same rules, that's my justification). In the same manner that Tink gave a "No Cuck" option for Mia, I would like a Step-family option for the mother & sister. I dont think it would be hard. Just write a slightly changed script where in the official version they are blood mother & sister of MC. In the other have it blood mother & daughter who are MC's step family, mum just married dad & brought daughter with.