AITA for Telling My Brother's Fiancée the Truth About His Past Right Before Their Wedding?

So, my (20F) brother (31M) is getting married in two months to his fiancée, Sarah (28F). Sarah is a wonderful person—kind, smart, and honestly way too good for my brother.

I love my brother, but he has a history of being a serial cheater. Every relationship he’s ever been in, he’s cheated, lied, and manipulated his way out of accountability. I’ve always warned him that karma would catch up to him, but he never seemed to care.

When he met Sarah, I genuinely thought maybe he had changed. They’ve been together for three years, and as far as I know, he’s been faithful. But last week, I found out that wasn’t the case. One of my close friends confessed to me that she had a “thing” with my brother six months ago. She didn’t know he was engaged, and when she found out, she cut him off. She felt guilty, which is why she told me.

I was furious. Sarah had been so kind to me, treating me like a sister. I couldn’t just pretend I didn’t know. So, I sat her down and told her everything—his past, his affair, and the fact that he was still lying to her. She was devastated and ended up calling off the wedding.

My brother went ballistic. He told me I had no right to interfere, that it wasn’t my place, and that I had just “ruined his life” over a mistake. My parents are also mad at me, saying I should have let Sarah find out on her own. Some of my friends say I did the right thing, but others say it wasn’t my business and that I should have stayed out of it.

So… AITA?