I walked in on my boyfriend and my little brother...

(throwaway account; and for the sake of this story I will be using fake names for obvious reasons)

so me (M) and my boyfriend Joseph (19m) have been dating for 6 months, and our relationship has been a bumpy ride to say the least. we always invited my little brother Dom to join us whenever we went out and my boyfriend was always so kind to him and would buy him whatever he wanted. everything was going fine up until a few weeks ago when I realised that both of them had started hanging out without me.

at first I thought nothing of it, until my brother stopped coming home most nights. he would say that he was simply sleeping over at his friends houses but the friends that he would mention wouldn't even be in town at the time. I confided in my friend Mary for advice on the situation, who told me to put a tracking app on my brothers phone to see where exactly he was going.

I soon found out that my brother had been sleeping over at my boyfriends house and I went over there to directly confront them. I snuck in through the back door (my boyfriend is lousy and never locks it) and walked up the stairs to Josephs room. I opened the door but I didn't see anything and realised I was being paranoid and my boyfriend would never cheat on me, let alone with my own brother.

I decided to leave and felt like an idiot for every doubting my boyfriend. As I was making my way out of the house, I noticed noises coming from the garage and decided to check it out as I thought someone had broken in. To my surprise the garage door was already open, now I was really worried. As I entered the garage I saw my boyfriend in the car, and it looked like he was cheating on me. I ran up to the window and saw Dom underneath him, a tear ran down my leg and I screamed "HOW COULD YOU DO THIS TO ME." I ran out of the house and didn't look back.

I'm now hiding out at Mary's house devastated and I don't know what to do. I never want to see either of them again.