They had a one night stand. They meet again but the guy is dating his twin sister. He still loves him, even if he's with his sister~
I like this he's being greedy, he knows his sister is going out with him but he knows who he really wants is him. In other stories they usually end up giving up on them and sad and crying blah blah blah~ But he's actually standing his ground. His sister knows, and maybe she's doing it on purpose so I guess it wouldnt mean theyre cheating but the sister might've tried to come between them. Either way he better tell his sister.
I Fell in Love with My Sister's Boyfriend