Funny, because I was actually hoping there would be some MFM action coming up in the future, maybe even some orgies. Rachel's dislike for James in the first few days could take an interesting turn after you DP her.
Ofc I think it should stay optional, but in a sense I think the MC "rewarding" his sisters' open-mindedness with some more dick in their lives would add some realism. Even they could bring up the topic. I mean, currently the whole thing feels a bit too much like a fantasy.
I don't mind all the fucking around, or anything, and I actually think the characters feel quite realistic for a game of this genre. That's why I feel like it'd add even more realism to the scene if it wouldn't just be MC's dream-world, where he gets to shag every hot girl, and even keep a harem of his own sisters.
Although I feel like
@Perverteer is going for polyamoria, it inevitably brings up the matter of sharing your loved ones, and once that ice is broken, you can look at sex in different way. Just as MC doesn't fall in love with every girl he beds, sharing your sisters later on could be a turn-on even for them, once they adjust to MC's way of thinking.