Am I overreacting? I told the girl who was being cheated on by my brother’s best friend about my brother’s affair with her man

My brother, Aiden, 21m was always really close to his best friend, who we’ll call Tom, 25 (soon to be 26) m. They’ve known each other since Aiden was 14 and Tom was 19. I always found that weird and Tom likely groomed him. Tom has a girlfriend who we’ll call Heather, 21f. He’s known her since she was 17, him 22. They started dating since as soon as he turned 18. He isn’t really that great of a bf to Heather, usually blowing her plans off and never taking her feelings into consideration. No compliments, not many dates. He just used her as a beard to cover up. My brother and I, 17f, were always really close, but now he’s asking me a huge favor. I walked in on Aiden and Tom kissing, and was extremely mad. Heather is a lovely girl and doesn’t deserve this. Apparently this has been going on for a few months. I confronted my brother and he begged me not to tell Heather as it’s so hard for him to find a lover as he’s gay. I thought about it and decided to tell her. I gave him a few days to come clean but he never planned to. Now my brother’s bsf is sending me angry texts calling me an asshole, a betrayer and backstabber, homophobic, a hoe, a slut, and threatening to kill me. He ranted about how girls never understand and Tom’s saying his parents are probably going to disown him. I feel bad for him but cheating is wrong. I have apologised and never meant for anything this dramatic to happen to him. His girlfriend is deeply in love with him and does not suspect anything, and Tom and Aiden had no intention of telling her. She was talking about their future marriage and Tom was agreeing and they’ve discussed their future a lot. I only told Heather and his parents somehow got to know. My brother’s been out for a while. My parents took his side and are super mad at me. They say I overreacted and and should have thought this through. Did I overreact?

edit: I didn’t name Aiden or the gender of the cheatee, she asked him and he spilled it to her.

edit: A lot of people seem to think Aiden had a gf, he didn’t. He was single.