Wife's Little Sister

Okay, my wife wendy has a little sister called Sally, they are very close and do everything together....anyway Wendy is 23 and Sally is barely 18....in some ways wendy is like a second mom to Sally....
but Sally is aways at our place, which I don't mind as she is good fun and Wendy just loves having her over.
This particular morning Wendy was getting dressed for work she is training to be a doctor, so she gets a lot of the weekend shifts, as she is dressing Sally comes in our bedroom and jumps in bed beside me in her usual fun way just messing about and tickling me...Wendy laughs at her sister and carries on dressing I watch her....her firm tit's and round ass, long dark hair....dam see she is hot....in my distraction I hadn't noticed that Sally had reached over and just as I looked at Sally she grabbed my cock while we are under the bed duvet...
As wendy and I sleep naked I was still naked when Sally started to wank my cock off, as she talked to Wendy who had her back to us and still naked as she picked out her work clothes.
Wendy said she was having a quick shower and went into the en suite, Sally quickly mounted my cock, I protested but not loudly as I didn't want my wife to see this.....Sally continued to ride me until I had cum deep within her....I was shocked and nervous to say the least
That 12 years ago...and nothing came of it...except my wife confessed her sister and her had planned the whole event