GIRLFRIENDS SISTER

I remember back when I was 18 and out of high school. I was working at a fast food place as a cashier. Thats where I met my girlfriend, she was a customer. Anyways she was a regular and the reason she was a regular is because of me. Yeah she had a crush on me, she was also 18 and had recently come out as lesbian. After a couple months of us "meeting" at my job she finally asked me out after my shift one day. I said yes, held her hand and started to walk home with her. We dated for a couple months before she took me to her house to meet her family. It was awkward as her parents were not exactly happy about her being lesbian. Needless to say I only went over when the parents weren't there. Most of the time it was just us 2, and yes we had sex a lot!! A small handfull of times her younger sister was there. She was 2 years younger and an introvert, thus very quiet. She was a small thing, about 4 foot 10 and 90 pounds if you were lucky.
I mention her because 1 time she was there, and we forgot, we had sex a couple times over an hour or so. My girlfriend got up and dressed because she remembered she had to help a neighbor with a couple things. We kissed annd she left. A couple minutes later her sister walked in blushing. Come to find out she watched us have sex. She also said that it turnned her on, but she didn't cum. She had said that there was a classmate she was attracted to but didn't know how to ask her out or even kiss. She asked me for help, and I was surprised. Thought about it for a little and as I heard her sister (my gf) walk in the house, I kissed the younger sister on the lips and told her I would help her. I gave her a hug and said it woulld be ok.
Needless to say over the next 3 months my gf & I still dated and had sex, and once or twice a week I would also be with her sister. By lookks they were different, even shaving (younger did not). And yet they both acted the same when sexually turned on, that actually made me hornier knowing the same thing turned both on!!