The only question is how Tommy and Derek will react to this. Derek knows that Josy/Maya and the MC are trying out the threesome, but he probably doesn't know that it's very far along. Maya and Derek do talk about it, but there are boundaries for siblings that they don't talk about. I don't think I need to elaborate on that.
I think Derek has gemekrt at Juno Swimmining and Spa so there is more between Josy/Maya/MC and he is afraid of being excluded from some activities as a DIK brother, brother and friend. This is why the dorm party is so important to Derek, because it gives him the time and validation from his MC brother that is important to Derek. But Derek also has to learn to accept that the MC is different. Therefore, in Derek's revenge on Ron, the neutral decision is actually the right one. The MC doesn't stop Derek from getting revenge and shows him that he accepts Derek for who he is, but the MC shows Derek that he doesn't want to do crazy things anymore.
The MC is a decent DIK brother and that qualifies the MC in Tommy's eyes to be allowed to have a relationship or throuple with his stepsister Josy. But with Tommy, it becomes more difficult because he definitely doesn't want Josy and also Maya to come to the DIK mansion permanently and spoil his fun (cheat on Heather). Also, Josy and Tommy hate each other. I don't think Tommy has anything against the Throuple per se, he's just afraid Josy will reveal something about his activities. That's why it's not good when Heather finds out about Tommy's cheating in EP7. If there are two pairs of legs in Tommy's bed, then the lighter pair of legs is Heather's leg. Of course then Tommy has had sex with two girls and should be happy, but it also lets him know that someone has cheated on him. And Tommy might suspect Josy behind it. But the MC can change Tommy to a certain extent. The MC can force Tommy to write PLEASE in EP8 when Tommy asks the MC to take the job at the Pink Rose for him. Also, he ends up getting the jacket for Heather from his room. I think the MC also has to convince Tommy so that Josy and Maya don't betray Tommy.