I finished in my best friends sister

So me (20m) and my friend (20m) had come home from a night out. He went straight to bed, and crashed. I was sleeping on his couch. I went to the living room to wind down. His sister (18f) was in there watching tv. She'd had a few drinks aswell. We got to talking, and I found out she'd had a crush on me for a while. I hadn't known at all. We got close, and eventually ended up in her room. We went for a while, and I finished without a condom or anything. I slept in her bed, and woke up the next morning, snuck downstairs to the living room, and pretended it slept on the couch. What do I do?