She is most definitely involved with someone else, and by the feel of it, not exactly of her own free will...wouldn't be surprised if the same mystery guy that's got his family under his thumb, also has her under his influence as well.
I bet that's how the MC was put into a coma, the guy that is manipulating (having sex with) his family and girlfriend tried taking him out of the picture, so he can have them all to himself.
Judging from the pace of the story, we'll get a much clearer implication of who the person is that's forcing his girl and family into sexual relations.
Edit: if you think about it, it makes sense that Elena hates the MC so much, because of something the MC did before he was put into a coma, (probably was friends with the guy that was manipulating the MC to get closer to his family and girl)...because of the MC trusting someone, they're all basically sex slaves for the creep...would make sense with the family and girlfriend being so reluctant to tell him what happened.