Does this make me a bad friend?

Ok so the other day I spent a night at my friends house. I woke up in the middle of the night to go use the bathroom when I saw that his sister's bedroom light was on. Through the crack of the door I saw her masturbating. I swear, she was really into it. I tried to get a better look by opening the door slightly but It squeaked and she caught me. I thought for sure she was about to scream her head off at me but instead she smiled and asked me "Do I turn you on?" I slowly nodded yes and from her drawer she pulled out a condom. The next thing I know we're having sex. It was the best sex I had ever had. I'm 18 & she's 20 so there's nothing wrong with that right? & I feel kinda bad that I had sex with one of my best friends sister, should I tell him?