Becca catches Anon with her mom. If he's already fucking Becca, she tells Tina. Becca tells him to stop fucking her mom, and doesn't find out if he continues to do so. Tina helps him cheat on her daughter by making her leave the apartment when they schedule sex. Becca doesn't know, and Tina doesn't feel guilty, so no one demands a branching option that forces the MC to choose one or the other.
Anon can fuck Jenny and her mom, but they never find out about each other. Jenny finds out that Anon is fucking Diane, but she doesn't care. Since none of the three women feel bad, no one demands a branching option for Jenny, Debbie, and Diane.
Anon is cheating on every woman in the game, but no one insists on a branching system that will force him to be faithful to just one of them. Grace gets singled out because she feels bad about having sex with her sister's boyfriend. Based on no one caring about Anon sleeping with both Becca and Tina, or both Jenny and Debbie, I conclude that no one cares about the MC fucking members of the same family unless one of the women feels guilty about it. So, don't bother rewriting an entire major story line. Just make it so that Grace doesn't feel guilty! Everything would be okay if Grace didn't feel bad, right?