As we reached a wooded bit of the park Sally stopped us."I'm so sorry, I really need to pee. Can you wait here and keep look out? There's a few bushes...over there. I really can't help it." That's fine, we won't peek" said Samantha jokingly.Sally trotted off to the bushes about 20 metres from us. Samantha and I turned our backs and looked around to make sure no one was coming.Within a few seconds Samantha tugged at my arm and got me to turn round."Look", she said. I could see Sally facing away. Last Tuesday, she was out shopping and I got home before she did. I was feeling kind of horny and decided to take matters into my own hands. I stripped down and lay down on the bed and was taking a good long time stroking my tool. I had the lotion out and was replaying a few moments from the Spank Bank back in my mind.I am so used to living alone, I guess I lost track of my surroundings because my sister walked in on me, lubed up cock in hand, lying back on the bed in my own filthy little. With one hand she viciously pinched an erect nipple, the other was busily flicking at her swollen clit. Her glittering black eyes were focused on the man’s stirring cock with a hunger that unnerved him. Delighted incredulity rendered him just as powerless as her incredible mouth had only moments before. She pouted prettily at his motionless state, then quirked the corner of her mouth up in an impish grin. In a smooth movement that would make the most practiced stripper jealous, she was leaning. I could feel my knickers getting damp at the thought of being bent over her lap.I realised she was probably interpreting my silence as an indication of my refusal. She looked suddenly doubtful, and her frown was back.‘I think I have just as much right as the Headmistress to punish you for being such a useless klutz. What do you say, Miss Prissy-pants?’I was startled. Could she sense my feelings? My desire? She didn’t sound quite so sure of herself as she had a moment ago. I wondered why. And I.