I hooked up with my best friend's brother while she was asleep in the next room

I don't think I can ever look her in the eye again and I need to get this out before it eats me alive.

I'm 22, my best friend L and I have been close since secondary school. Her family basically adopted me - I spent more weekends at their house than my own growing up. Her older brother K is 25, recently moved back home after breaking up with his long-term girlfriend.

Last weekend was L's birthday. We got absolutely wasted at her parents' place - they were away on holiday, just us and a few friends. Everyone left around 2am except me because I was too drunk to get home safely. L passed out in her room and I was on the couch in the living room trying to sober up enough to sleep.

K came downstairs for water. We started talking, he was being sweet checking on me, and suddenly I noticed how different he looked. I'd known him since I was 13 but in that moment he wasn't "L's annoying brother" anymore. He was just... a guy. A really attractive guy who was looking at me like I wasn't his little sister's best friend.

I kissed him first. He hesitated, said "L would kill us," but I pulled him onto the couch and he stopped arguing. We had sex right there with her bedroom door ten feet away. I had to bite his shoulder to keep quiet because the walls are thin and she was right there.

He was gentle, almost too careful, kept checking if I was okay. Afterward he held me and said this couldn't happen again, that he cared about me but L would never forgive either of us. I pretended to agree.

L woke up the next morning and made us pancakes while we sat at the table acting normal. She commented on how quiet we were and we both blamed hangovers. She believed us.

I've been avoiding her texts since. Not because I regret it exactly - it was genuinely one of the most intimate nights I've had - but because I know I'm going to do it again. K texted me yesterday saying he can't stop thinking about it. I know I should shut it down, protect my friendship, be a good person.

But I want him more than I care about doing the right thing and that makes me feel like a terrible friend and an even worse person.