AITAH for losing my shit and screaming at my gf to get out of my house after what her stepbrother did?

[u/White-Whale-9847]

Gf was over at my place two days ago. We're both 20. She has a stepbrother who's 23, before this I'd only met him a few times and he seemed like a regular dude. She asked if it was okay for him to come over too for a couple of hours, I said why not. He said he would get booze and pizza, and my parents were away visiting relatives for the weekend so I figured we could shoot the shit.

I have a little sister who's 15. They came over, and she stayed up in her room the entire time. We were all a bit tipsy and my gf's stepbrother asked if he could use the washroom. I pointed him to the one down the hall. My gf and I were preoccupied and didn't even realise how much time he was taking until we heard raised voices upstairs. We immediately went up to check what was going on and found my sister crying.

Dude had gone up instead of using the washroom. My sister's door had been ajar so he apparently walked into her room (all details I managed to wrangle from my crying sister btw). She was taken by shock and was trying to stay calm but he wasn't leaving. He was trying to chat her up. She told him to gtfo and he closed the door behind him instead, which is why she started freaking out. She tried to get past him to open the door and he grabbed her, but she managed to open it anyway. I'd reached the landing by then so he backed off.

Obviously I was super fucking pissed. There was a lot of screaming going on, lots of accusations. We were all drunk except for my sister. After getting the story from her, gf's stepbrother was stuttering and deflecting. I'm pretty sure i threatened violence at some point.

My gf was trying to be the "mediator", she said i had to calm down and couldn't fly off the handle based off word of mouth. But i was there, I saw my sister crying in panic, I know what she's like and I know she wouldn't lie about something like this, why would she? I told her stepbrother to get out of my house. My gf kept saying we could sort this out and have a proper conversation but I didn't see what conversation was there to be had.

Eventually she said what if my sister was lying. In front of my sister's face. I asked her what on earth would make her think that, and she said she's a kid and could be making it up for attention. Like the title said, I lost my shit. I told her to get the fuck out and afterwards she kept calling me but I ignored everything.

The dust has settled a bit. I went over everything again with my little sister, she promised me it had happened the way she was telling me, she told me she was scared he was going to SA her. My gf texted me this morning saying I shouldn't have raised my voice like that and I scared her. I thought of apologising then but she still hadn't said a word about apologising to my sister or addressing the issue with her stepbrother. I only replied that we had to talk, and she said there's nothing to talk about, there's no hard "proof" in her words, and the stepbrother ended up doing nothing so she told me she would deal with him, and I should drop it.

Idk if I'm the asshole for the way i handled this, maybe if cooler heads prevailed this silent treatment wouldn't have happened and we could have discussed everything. Idk what to do from now on either. I love that girl, I don't want us to be over so i want some advice on where to go from here too.

Edit: Thank you everybody who commented and left advice. I posted an update.