A threesome with my sister...?

My sister was once again visiting with her new horny boyfriend, who never misses an opportunity to stare at my tits and ass ... My sister is already embarrassed... He actually suggested that he wanted to have a threesome with us!!! Really cheeky of him!!... but still, my pussy got wet immediately at the thought, so I disappeared into my room to do it myself. I was just about to... I imagined what it would be like to feel his thick cock inside me.... Then a cleft of the door opened and all I saw was a thick cock and I got even hornier! Then my sister's boyfriend came in, even though my sister was watching TV in the next room. He didn't care, or so it seemed to me... before I could say anything I already had his cock in my mouth. You won't believe what happened next.....!