Younger sisters girlfriend

The week after Christmas my younger sisters girlfriend stayed at our house when her parents went away for the week. On new years eve my parents went to a hotel party and were going to stay overnight. I went out to a party also and got home about 2 am. When I got home it was obvious my sister and her girlfriend were drunk. Both could hardly walk and my sister had vomited on the kitchen floor. I was mad at her but did clean it up. I had to carry my sister up the steps and put her in my room and her girfriend staggered to bed in my sisters room. I made a sandwich and watched tv for awhile and was going to sleep in my parents bed. Went I went up stairs I took a bucket into my room in case my sister got sick again and grabbed a trash can from the bathroom for her girlfriend, thinking she might get sick also. When I went into my sisters room her girlfriend was sound asleep with the lights still on. She was on her back and the only thing she had on was her panties. I was suprised at the size of her breasts and just stood there admiring them. I got an erection right away and knew I should leave but didn't. She was obviously passed out and I pulled her panties off and just put them with the rest of her clothes. I opened her legs gently to see more of her and just looked at her for a long time. I couldn't stop myself and started to touch her breasts and for that matter her whole body. She never moved and I even took her hand and put it around my penis. I could see her open vagina and I started fingering her. After a few minutes she still didn't move but I could feel her getting wet. By this time i was so aroused I got between her legsand started to lick her vagina and hold her breasts. In a short time I could feel her pelvis start to respond as she was slightly moving her vagina back and forth toward my tongue. I could even hear her making sounds and knew when she had an orgasm. When I finally stopped I got up and masturbated into a towel. I turned out the light , closed the door and went to bed. The next morning when I saw her she seemed to know something happened. She didn't say anything but looked at me like she was aware of what I did. She avoided talking to me all day and avoided looking at me.. She went home that Saturday and I havn't seen her since. I'm just afraid now that she would tell my sister. And I know my sister would tell my parents right away. I never said anything to my parents about them being drunk and my sister did thank me for not telling on them. I can only hope now her girlfriend either doesn't remember or is to embarrassed to tell anyone. If she does know what I did maybe she just doesn't want her parents to find out she was drunk. I know I shouldn't have done it but at the time I was so aroused I couldn't stop myself.