Younger sister and girlfriend compete for my attention - I choose younger sister for sex

I have a rather unusual situation going on at my place. My younger sister and my girlfriend are both vying for my attention. My girlfriend is a hot 18-year-old babe who loves to play the field, while my younger sister is a cute 18-year-old with a nice ass. The other day, when I was out of the house, my girlfriend invited my younger sister over for some fun. When I came back, I found them both in my room, stripping off each other's clothes and having a good time. I couldn't help but get turned on by the sight of these two hot babes fucking. I decided to join in and take my pick. I went for my younger sister, who was more into it than my girlfriend. It was a wild ride as I fucked my younger sister, while my girlfriend watched on. This was definitely one of the hottest moments I've had in a while.