Well there has to be a good reason why Millie sleeps with Deb, even though she should know that will not help her get her in her dad's bed. You know it is not a turn on if someone sleeps with someone you are mad at as a male (never understood that female notion that making you jealous would work, it always just makes me pissed off and dump a woman) and it also feels like you take their side of rest family against MC making it alright what they did and besides if her emotional and sexual needs are filled by her female family members what reason has MC to give into her as he feels uncomfortable with it?
So MC must be the bad guy for it to still makes some sense from her pov or she has to be dumb as bricks empathically speaking.
Which may be true if we remember the Deb almost dying was a funny joke as well ......., something like that would have pissed me off so much I would likely have at best refused to talk with either for a few months after that if I had been the MC and that if is they were lucky and I was in a very forgiving mood. Which MC when comes to Deb was not.
From Debs pov, well she just more or less blew her last chance at redemption, ignoring MC's request to stay away from his daughter while still pissed at her, not forgetting if he has problems with the incest part when comes to Millie, he will feel no better about the incest of Deb with Millie, whether she is female or his sister will not make that any better. Instead of showing some deserved remorse for her actions in past, she forgoes MC's wishes completely again not taking his wishes into any consideration and repeating her patterns of past. Big major fuck up.
For neither it is a smart idea if want to get with MC, but then again who has smart ideas in this game.