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The original was posted on /r/AmItheAsshole by /u/throwaway929119 on 2023-09-06 02:11:56.

My brother (32M) and his girlfriend (28M) have been together for eight months. I knew the girl before my brother her, we weren’t friends, we didn’t chat much, but we were in similar groups of friends.

The girl has had serious mental healt problems before. She also has confidence issues. After she met my brother and started dating him, I can say that these problems diminished. He had a good effect on her self-esteem problems.

So we were talking two days ago and he told me that his gf cheated on him. When I asked him what he was going to do, he said that he wondered if she would confess to him, so he didn’t confront her. He was planning to break up when she admitted it. I don’t know how he found out about the cheating but it happened a week ago and since then girl has been at home depressed and crying. I wasn’t surprised because I knew she had previous mental health problems and was an emotional person.

I told my brother that she probably felt guilty, she was upset because she didn’t know how to admit it to you and I think he should end this. I told him that it’s cruel to do such an “experiment” because of her history of mental health problems. He told me to mind my own business.

That night my brother was out. So I went to his house and told her that my brother knew everything that their relationship was over. I helped her pack her things (she was staying with my brother) and drove her to a friend’s house. I think that’s what I should have done.

My brother called me today and told me off. We had a fight about it. Sometimes I really think my brother is a sociopath but I’m not sure about involving in their relationship. AITA?