After talking to my girlfriend about it she let me known that her sister was getting horny watching us on the couch.
My girlfriend told her that she would let her sister masturbate quietly under a blanket in the basement if she got too horny watching us. She said she was a 75a bra, (spelling mistake obviously? I asked her name, to which she said "just call me ___" and I don't know if it was the name of the video or her actual name.
One night her younger sister came downstairs quick , we were surprised as we did not hear her coming and my fingers were already massaging her clit.
She gave me a look that met to not stop as she was close from orgasm.
When leaving their house often late at night I would use the basement door.