My wife and I visit her mother quite often, as she lives nearby. My SIL (30F) lives with her mother at the moment after going through a rough patch, so we see her every time we're over there as well.

Lately, she's been wearing these short little dresses around the house, that leave very little to the imagination. I've tried being good, but I can't help but sneak a peek whenever she walks by and my wife isn't paying attention. She's got amazing legs that lead to a tight ass that I like to imagine isn't wearing any underwear.

The other day, we were over there and my wife and MIL were outside doing some gardening. I was sitting on the couch when my SIL walks in, and just sits across my lap, straddling me in one of those cute dresses and puts her arms around my neck.

I'm surprised, to say the least, but my cock is instantly hard and pressing against my pants, against her thigh. The thought of being caught like this is a surprising turn on, and after a second I put my hands on her waist.

She tells me that she's seen me checking her out, and it's been driving her crazy. She's been waiting for a quiet moment to do something like this, and she kisses me, hungrily, like she's aching for it. I miss her back, still half paying attention to the door in case someone starts to come in.

She grabs one of my hands, and guides it down between her legs. Sure enough, she's not wearing panties, and her pussy is soaking wet as she grinds against me. I'm completely losing control now, I don't care anymore if we get caught. I slide a couple of fingers into her pussy, and start to play with her as she keeps grinding. Her hand falls to the bulge in my pants that is my cock, and starts to stroke it, just teasing me. I want nothing more than to throw her down and show her this cock that she's playing with, but I control myself, and let her just play with it.

Her breathing gets faster, then she shudders and moans as she cums. She kisses me even harder for a moment, then leans back, gives me a goddamn sexy smile, and slides off of me. She leans in and whispers in my ear "next time, it's your turn", then gets up and returns to her room.

I sit there for a moment, still kind of processing what just happened, before I hear my wife and MIL coming back in. I get up and go to the bathroom to clean myself up, then come back out like nothing happened.

That night, I absolutely railed my wife while thinking about her sister, and what I want to do to her next time.

Hopefully it's soon.