AITA for ruining my sister’s wedding after discovering her fiancé’s secret?

I (30F) have a younger sister, Emily (26F), who’s always been like a best friend to me. We’ve shared everything, and I’ve always supported her, no matter what. So when she got engaged to Alex (29M), I was thrilled for her. Alex seemed like a great guy—he was kind, charming, and they were absolutely perfect for each other. I genuinely thought they were a great match.

However, about a week before her wedding, something happened that I never expected. I was at my mom’s house when Emily came over to visit, looking visibly upset. I asked her what was wrong, and after some hesitation, she told me that Alex had been acting distant lately. She had caught him texting someone repeatedly and had noticed he was hiding his phone more than usual. At first, I reassured her, telling her that every couple goes through rough patches and that it probably wasn’t anything serious.

But when Emily mentioned that Alex had gone on a “business trip” for a weekend and came back acting strange, I started to feel a sense of dread. I told her I would help her figure things out and offered to talk to Alex if she wanted me to. At first, she refused, insisting that it was just her insecurities. But then, a few days later, she called me in tears and said she was having doubts. She wanted me to talk to him for her, and I reluctantly agreed.

So, I took matters into my own hands. I confronted Alex under the pretense of "having a casual chat" and asked him if everything was okay with Emily. At first, he denied that anything was wrong, but when I pushed him further, he admitted something that made my blood run cold. He confessed that he had been seeing someone else for months. He told me it had started as a fling but had turned into something more serious. And worse, he had planned to keep it a secret until after the wedding.

I was absolutely shocked. I couldn’t believe what I was hearing. This man, who I had always thought was perfect for my sister, had been lying to her for months. The fact that he had been planning to marry her while hiding this secret made me furious. I felt like I was in a nightmare, and I didn’t know what to do.

I couldn’t just sit back and watch my sister marry someone who had betrayed her. So, I did the one thing I thought would wake her up to the truth: I told her. I didn’t hold anything back. I told her exactly what Alex had confessed to me about the affair and how he had been lying to her. She was crushed, understandably, and immediately called off the wedding.

The fallout has been huge. Emily is heartbroken, and I feel awful for breaking her trust in such a dramatic way. She’s not speaking to me, and our family is divided. Some family members think I did the right thing, while others say I should have kept quiet and let her figure it out on her own. Even though I know I did the right thing in the long run, I still feel guilty for ruining what was supposed to be her special day.

So, AITA for telling my sister about her fiancé’s affair, or should I have kept quiet and let her marry him, even though I knew what he was hiding?