Because unfortunately, none of the characters are bisexual. So yea, they're predisposed to being attracted to members of their family, but it only really works with people of the gender they're already attracted to.
Max is straight, she's only into guys, so she was only attracted to MC. Also, lots of people IRL have crushes etc on cousins and other family members, so her having feelings for MC like that isn't unusual genetic predisposition to incest or not.
The mom and aunt are straight, so they wouldn't be attracted to each other.
The dev has said there might be some girl on girl action in the game, but given how painfully straight the characters are, it's almost assuredly going to be purely for MCs benefit. Like say seeing one of his sisters eat out the other would be hot for MC and turn him on, but it wouldn't be something the sisters would ever do without MC present