<human>: Can you condense the text into three sentences or less?

When this school year started, 18/f began to like 18/m. But then she realized that he just came off as flirty to everybody, and he had a past of drug use and whatnot so she decided to "hate" him... (naiive, freshman girl thinking, I suppose.. I don't know).

Then without meaning to, 21/f (who happens to be 18/f's sister) got close to 18/m through mutual friends. They hook up a few times, and now like each other. 

18/f finds out, and confronts sister (21/f), telling her to stop talking to the guy. 21/f tries, but realizes she really likes 18/m and she can't.

Just tonight, 18/f tells her sister that she's actually really jealous that 18/m never liked her in that way, so 21/f feels very guilty about the whole situation.

WHAT NEEDS TO BE DONE?! HELP!
<bot>: older sister is in a relationship with a guy her younger sister used to like, and now the younger sister is asking her to stop the relationship because of her jealousy.