The way the game was going initially, I have little doubt that Eric was going to have sex with Lisa. Maybe it was that very night. Both Lisa and Alice deny doing stuff with Eric, even though that was the clear implication of the scenes. I suspect DS went back once the backlash began over Eric and the denials came from later updates. Or it was Lisa lying about it, and Alice being too drunk to remember.
Ann was in the room, but I don't think she would have tried too hard to stop Eric. He doubtlessly would have talked to her about it before, and talked her into it. The problem with Eric was that the entire family took his side in everything, and no one sided with him more than Ann. She would give him Lisa in a heartbeat if he put any pressure on her, or threatened to leave. With Max, she apparently likes and trusts him far less, plus there is the incest thing that's a big barrier for her when it comes to penetrative sex.
So I think Eric either did have sex with both Lisa and Alice (depending on what choices you made), or at the very least was headed that way before fans reacted and DS changed his plans. Fortunately, there were never any renders that show this conclusively, or dialog that confirms it 100%, so it does leave room for some doubt as to what really happens. I prefer thinking that Eric never touched Alice or Lisa, and ideally he never would have been in the game as he was in the first place.