To be fair Bingly we don't know Sean is cheating on her, just that he's looking at tattlers socials. But yeah she's lying about a lot of stuff.
I think she would be more inclined to cheat, she seems to like to get one over on people, Sean seems loyal to a fault. If he wasn't he'd be long gone. I think he feels responsible for her and not in a good way, but in a psychologically draining sense. And I think that is multiplied by her families manipulation of the situation, especially Nanas illness. I'm not even sure he's aware, it's all he's known as an adult. Lurking on someone's socials probably feels safe and he's not expecting that to be reported on, also if Demi has access to his socials she wouldn't know what he's viewing.....if she's read that here that answers the wedding question.