I don't see how the MC is going to be romantically involved with any of them realistically. I mean I know they will waaaaaaay down the road, because well...this is a sex game after all. But both girls have serious issues (especially the mom) only a professional shrink can help with, and the MC screwed up his last job for being involved with his client. To me, the Dev went a bit overboard with the trauma, anything involving physical intimacy with Charlotte should be impossible; Emma is a much better option (but again, he screwed up his last job for doing that, so realistically, no dice)...it feels though that the only seducing and play the MC is going to get for a long time will be LI's outside the house.
Lucy: Probably earliest. If it doesn't interfere with MC's work, I don't see why anyone would have a problem with it.
Charlotte: Really bitchy at first, but you can already see she's easing up. The game will probably need several plot points where she starts to trust/fall for MC.
Suzie: This girl has anger issues, but it could work, and she's nearly an adult. But it would really take some convincing with Charlotte.
Emma: I can't see this one. Maybe, somewhere at the end, but she's too innocent as it is. ( or more suited for Mr. Dots weirdos
). Really works best as daughter figure.
Any threesome/harems are probably out of picture ( except if he adds any more love interests outside of family, and with Lucy).
This game lives or dies in how good relationship/character development is, so honestly worst thing would be rushing it.