A couple is visiting his parents for a week or two. Things have been rocky between them for a while. But they adore her. Of course, they can't really hide how bad things are between them. After a fight he leaves.
Npw his sister or brother turn to her to comfort her. It's just a friendly way of comfort and she really knows his brother/sister well. But after yet another fight, she breaks down almost and comfort turns into more. Her brother or his sister end up sleeping with her. It's awkward in the morning but neither of them regret it. Of course, they both try to hide their affection. But they can't help it....
Both ways will be confusing as she never thought she'd be into women OR younger men. I was thinking her boyfriend (or rather ex) is in his 40s, the younger brother around 20.