My brother knew about my teen sister's domestic relationship and did nothing.

My sister is a teenager; all I will say is she's in high school. She was in a relationship that had many toxic incidents, but she didn't tell us it was abusive till a bit after they ended it. Tell me why I just found out that my brother has known about it and didn't say anything, and didn't do anything to help either. My sister is so young, and he said, "It wasn't my place to tell. I don't have to tell everything" he is an older brother, and does not think he did anything wrong. Hes known while my sister was in the relationship and he was still all buddy buddy with her partner up until the end. Knowing her partner was beating her and I am furious and hes acting like I have nothing to be mad about.