Saw girlfriends sister naked help me has this happened to anyone else?

I was swiming with my girlfriend and her older sister today in her pond. Then my girl frend told me to meet her in the kitchen and she told me her parents just left and wont be back all weeked and said i could stay the night if i never tell her parents. So we started to make out on the counter and then she felt my dick get hard so she your excited arnt you and then her sister walked by looking at us like a pervert and said have fun i wont say anything. And they look very similar long brown hair skinny and similar faces btw then she said i have to take a shower and my firlfriend said she did to so she told me to go to her room so i did but then i thought it would be funny so when i got in there it was her sister and she was masterbating and saying my name and staring at my penis and i ran out into the other bathroom and told my girlfriend what happed and she said dont worry she said her sister masterbates alot and to alot of things so dont sweat it but later that night i slept in my girlfriendds room and we were having sex buti could not get completly hard and my girl friend didnt really seem to notice because she was having a good thime and she was riding it so hard but i could not *** because i kept thinking about her sister masterbating to me and i got a mental block. In the morning her sister came down the stares and said way to go sis on taking that monster last night she said how did you know and she said i could hear you moaning and the screaming all night