I'll try to explain it as best as I can. I live with my dad, stepmom and stepsister M. She's 18 and I'm 20, this weekend we are home with the two of us and she decided to throw a party yesterday.

I went out to a buddy of mine for a smoke sesh and came back around 11 P.M. M was having a girls night and she invited 5 people. I have seen all her friends a few times and I would consider myself to be good friends with M. I greeted everyone and went to my room as to not disturb them further. Was feeling pretty good already but decided to roll myself a blunt and took my bong out for a few bowls throughout the night. 

Around 12 the munchies kicked in and I went downstairs to grab some snacks. They saw me in the kitchen and it was obvious they were already pretty drunk. I got dared to take 4 shots back to back and they started teasing me and calling me a pussy for not doing it. Now I was already pretty high but decided to to do it because why not. M asked me if I had some green and I gave them my bong and took a few hits with them. Left the bong downstairs for them to use and went up with my snacks and rolled another blunt. 

I was just puffing and watching YouTube in my bed when someone opened my door. It was M and her friend S. They were laughing their asses off and they both jumped on my bed. I was roughly a [9] at this point and was not sure what to do. I asked them how they were doing and all they did was giggle, touch and tease me. They were both in lingerie too. Somebody called M and she left my room but S was still laying down on my bed. Usually she's pretty shy but a real sweetheart nonetheless. That was not the case now and she was barraging me with questions and kept eying me. 

Long story short, we shared the blunt I had left and went to watch some Netflix. This eventually escalated and we ended up having sex and falling asleep afterwards. I don't even know if we used protection.

This morning I was woken up by M who angrily told me to 'wake the fuck up'. She told me she couldn't believe I had slept with her friend and called me disgusting. I tried to apologize but she was having none of it and stormed out of my room. She hasn't talked to me since and acts like I don't exist. I have a bunch of messages from S thanking me for the night and that she loved it but she had to tell M and that she's sorry.

Am I the asshole here? Honestly if I was fully sober I would have never done this but I was not in control. I definitely understand where she's coming from but I don't believe I'm entirely in the wrong here.
