I (27F) found my cousin (24M) spooning a 15-year-old girl in my parents’ house. I tried to kick him out but my family is siding with him. What do I do?

ThrowRA because my family knows how to find my Reddit account. Posting here because r/AITAH didn't give me much advice on what to do next.

This happened last Friday/Saturday and I'm still fuming and confused by the reactions of everyone involved.

So, context. I (27F) am living at my parents' house and my cousin (24M) came to stay here last Friday. He'd moved to another country a few years ago so this was our first time seeing him in quite some time. He'd been like a brother to me all my life so I was initially looking forward to hanging out with him, and he wanted to contribute to bills while he's here anyway. I then found out from my mother that he'd be bringing a girl with him who's working with him on a project, and that they'd be staying here as a change of environment and to chill out. I thought “pretty weird, but all right,” and didn't question it further.

They arrived Friday and I found out why my mother said “girl” instead of “woman”. His “coworker” is 15. We were all shocked upon learning this and there were a lot of awkward silences at first, but he explained that she was working with him on a game and that her parents had given the green light for her to travel with him. Apparently she and him have been living under the same roof for around a year.

We collectively kind of just accepted it, laughed it off and left it at that. Internally though, I had a gross feeling which was further justified by the fact that they acted like they were on a honeymoon. She's very clingy to him, constantly gives him a sort of doe-eyed look when he talks. I originally thought it was just because English isn't her first language, but she speaks perfectly good English. Another thing I noticed was that she was wearing a necklace that was halfway between a choker and collar that when I asked her about it, she looked at my cousin, smiled, and said it was a gift from him.

So, fast-forward to Friday night and before we go for drinks, we're figuring out sleeping arrangements since there'd be a total of 8 people in the house now, including my siblings. Originally we were going to let my cousin and the girl (I'll call her A from here on) take the couch and airbed in the living room but after finding out A is a child, I offered to give her my room and I'd take the airbed, while my cousin would take the couch. She refused no matter how much I reassured her, and said she'd like the airbed. I tried suggesting a bunch of different ideas, including her having the living room to herself, but she was insistent on my cousin being there with her. I really felt uncomfortable about this, but my parents and one of my siblings brushed it off. My two other siblings were as weirded out by this as me, but what could we do? At least they were going to be sleeping seperately.

Everyone goes to the pub, except A, my cousin (neither drink) and my mother. She works Friday nights, so my cousin and A were home alone. They said they'd watch a movie and go to sleep. I tried to forget about it and just enjoy the night, but I couldn't stop worrying. I ended up drinking a lot.

Here's where it went from 0 to 100. A lot of the details are fuzzy from the beers, so bear with me. We all come back home around 2am and we find them both asleep on the airbed, spooning and cuddling. It felt like walking in on a crime scene, but at least that would have a more straightforward reaction. My Dad and siblings are whispering, discussing what to do next, but I just snap. I yell at my cousin and wake him up. I grab a cushion off of the couch and knock him back with it, and ask him what the hell he's doing. A gets up and wraps herself around him, and tells me to leave him alone. He steps off the bed and tries to calm me down, telling me to let him explain. I swat his arm and yell at him, saying that there's no just reason for a 24yo man to be cuddling a 15yo girl he's not related to. At one point I heard him say “we're wearing pajamas.”

Eventually I get him to the door, push him outside, lock it and take the key. A tries to take it and starts crying, all the while yelling at me to let him back inside. Everyone else is trying to calm me down, saying that I'm not thinking straight and that I need to sit down and sober up while they handle it. They convince me to give the key to A, who rushes to the door and brings my cousin back inside and starts hugging him. My brother and one of my sisters leads me to the kitchen while my Dad and other sister talk with A and my cousin. I don't know what was said, but they told me it's been sorted out and to head up to my room.

The next morning we talked during breakfast, and I found out that A and my cousin went back to the airbed after the argument. I got mad and asked why the hell they were still sleeping in the same bed, and my Dad literally said “they're just close.” When my mother came home and found out, she told me basically the same thing, and said that we've known my cousin for most of our lives, that we should trust him.

I still can't comprehend it, and yet everyone's moved on already. In fact, some of my family is annoyed at me about how I reacted. I feel like I'm in a Twilight Zone episode. I apologized to A, but she said to save it for my cousin, who I refuse to give an apology to.

What do I do? Is there anything I can do?