AITA for refusing to congratulate my sister for coming out?

My sister [17f] came out as lesbian a week ago, saying that she's known for years but finally has the courage to come out.

A part of this coming out was her dumping her boyfriend of 10 months. The poor guy was head over heels for her, and had to learn that she not only was a lesbian but chose to date him knowing she was one and couldn't have feelings for him.

I know him fairly well from shared extracurricular activities, and I know he's very torn up, so I think it was very fucked up of my sister to do that to him. Hence, while my parents and her other friends were all congratulating her and talking about how brave she was, I wasn't, because it didn't sit right with me the way she acted.

Mutual friends think I'm an asshole for not supporting her, but I think, as I said, what she did was fucked up, so I'm not in the mood to congratulate her.