Dear JB readers: My brother is still friends with my ex, is it right?

From a female LIB reader

I dated one of my brother's best friends for a year and two months and broke up with him just last November. My ex has moved on and so have I but what pisses me off right now is that my brother is still friends with this dude. I'm angry at that because our relationship ended because his friend cheated on me with someone in our inner circle. I expected my brother to at least have my back and be on my side on this one but he's still hanging out with my ex and even going out with him and his new girlfriend (not the girl he cheated on me with). I confronted him some days ago and he said what we had was between us and he never got involved and their friendship is between them and I shouldn't get involved. I'm currently not on speaking terms with my brother because I feel betrayed by him. I was hurt by his friend and he would continue to be friends with him? Where's the loyalty? I've cut him out of my life until he cuts his friend out of his life. My brother is wrong for still being friends with a man that hurt his baby sister, is he not?