Yes you will need to refer to the relationship posts for an understanding if you've never played any Inceton games before. Resort of temptation is like a cross-over fuck fest where they put the characters from previous games into 1 and see them fuck each other brains out. It's more fun to play with if you had played previous games or at least know what was it about.
As for the original games themselves,
I would not use the word "very little" sharing content, I would say there's none at all. If I remember vaguely from summer with Mia, there's a scene where MC and Mia play with their best friends but that's pretty much it. The story of Summer with Mia has always revolve around incest with the sister, and even by the extent of the finale Return to Mia it's a solely monogamy story. I would say they are really decent play them if incest is your kink. Return to Mia has MC to join in with another couple for fun, you can either chose to listen to them and just play along the usual 3some thing or cross the line and cuck the guy. It's just 2 or 3 scenes of content because the girl isn't 1 of the LI.
Shackbang is a short story game I strongly recommend everyone play it because I can't be the only 1 to get my brain fucked by the ending. The game is getting a remake btw.
Lust theory.... hot models, all I can remember up to season 3 was MC fucking the shit out of every female that has a name.