AIO for telling my Brothers girlfriend he has Herpes?

I (20F) found out my brother (23M) has herpes. He’s been dating his girlfriend, for a month but hasn’t told her. When I asked why, he said it’s “not a big deal.” I couldn’t stand the thought of her not knowing or getting herpes herself, especially since we’ve gotten close.

So, I told her. I tried to be gentle and she was understandably upset but thanked me for being honest. My brother found out, lost it, and called me a traitor and said I have ruined his relationship. I don’t know whether I did the right thing or should have let him tell her himself.

Did I do the right thing, or did I overreact?