My best friend and I visited his sister in college when we were seniors and she was a freshman, we went out to the bars with her and her friends (they all had fake ids). His sister was totally all over me all night. I continuously pushed her away (which made it worse) and explained to her that even though I thought she was really hot I would not do anything out of respect for her brother.

Her brother gets tanked and I got really drunk as well and I briefly made out with her at one bars without her brother seeing. Later in the night I crossed the point where my drunkness outweighed my friendship and I let her blow me in the backseat of a cab while her brother was in the front seat. (Still had no idea). Her and her roommate snuck us into their dorms were my drunk best friend and the roommate immediately passed out. I then proceeded to fuck her not 4 feet away from her brother. He still does not know. I am a terrible, terrible friend.