Well yes, depending on some choices you made earlier, Sandra IS okay with an open relationship, so the tension here is not just coercing her into having mutually agreeable threesome sex. That she is open to that is a given, she wants the Hellcat as much as MC at this point. It just took her a bit longer to admit it to herself. Getting her to the point she is okay with sexual relationships with her own daughters though, that's another hurdle, one I think that has been written quite brilliantly up to this point, and I for one would hate to lose that brilliance or have it played out too quickly (as I felt that was a flaw in Parental Love).
True, but suspicion isn't proof, and to me the notion that the girls suspect, but can't prove with 100% certainty is part of the appeal.
Except that was all just fun and games and nothing more serious than nudity, making out and heavy petting occured. As far as the main girls are concerned, they all just got drunk and did typically embarrassing party stuff. No worse than some of the stuff I did in college.