You're right! I totally forgot about that! Still, D is going to have to sneak into her Dad's room to fuck, either that, or Dad is going to have to visit Daughter's room. And fuck! What if Daughter & Elena share a room?
I could maybe see how that might work out: Elena catches Daughter sneaking out, she listens and watches while they fuck, gets turned on herself, starts frigging herself in the hallway, then somehow someone talks about the elephant in the room, Elena says she's cool with it or maybe blackmails the Dad into letting her in on the relationship?