It's a 2 part series. In the first one, (in a flashback) the sister tells her brother that someone confessed to her and that leads him to force himself on her. (I explicitly remember it didn't have the rape tag on it because i found it weird that it didnt have it). Later on, they do it again because they confess they're into each other. On the second, there's a timeskip and they just came from the convenience store and they do it on the couch. He mentions some people were checking her out cuz her outfit was rather revealing.