Brother dating ex who I just broke up with because she fucked the same brother. Am I right to be angry?

Me and my brother are 17 so we live in the same house which is the main problem. Two weeks ago I caught my brother ******* my girlfriend who I have been dating for a year. We got into a fist fight that my dad ended up braking up. Instead of talking I ignored them for about a week until I agreed to sit down and talk to them to let them explain. She said she got close to my brother over the last month as she was lonely because I was always busy with school and the clubs I was in like football and ruby. She then told me that it wasn't just sex and that she now has the same feelings for him that she had for me at the start of our relationship. I sorta accepted what they were saying and we split and I made up with my brother and everything was fine until the last couple of days where they would come back to our house and I would see them making out I front of me in the ******* living room. This led to another massive fight and I haven't talked to either of them since. Every time I look at my brother all I feel is resentment and seeing them together is the worst feeling ever. My dad is upset because the same thing happened to him and his brother but it was with his fiancée so he knows how it feels. They have tried to talk but this time I really have no intention of making up with him. Am I doing the right thing