I'm fine without her, not because son has already fucked her but that maybe she'll keep him away from the others.
Oh yeah I was just suggesting it from the point of view of the MC himself, I know I wouldn't want to bed my Sons or close friends GF, aside from being a shitty thing to do I would likely keep thinking of them which would kill my libido.
I'm not really interested in her as a "harem member?" mainly because I'm not keen on big breasted characters as a whole, that said though I am kind of interested in her story with the whole supernatural dream thing, the contract, who she is and what she is all about etc, that might be good.
But I'm putting this game aside and waiting to see whether or not it's for me, maybe that will become clear in future updates, what type of game is the dev aiming for and what the focus will be, a swinging game, harem game, something else entirely. I will hold off getting invested in the story until that is clearer.