Me, my gf, and her sister?

Ok so let me just give you a little back ground info. I’m a 23 year old guy, love working out so I’m in reasonably good shape and I’m 6’2” 200lbs. I’ve got a girlfriend who’s 19 and she has a gorgeous body, she’s 5’10” slim toned body a stunning bum, perky DD boobs, slim smooth legs and long dirty blonde hair. We love sex and we,re pretty adventurous with it.

Alright so this is an insane but very true story of what happened to me over the last few weeks whilst I was staying with my girlfriends family (brother 13, sister 16 and parents). I get on well with her family so it’s always fun staying at their house, but this time it was a lot more than usual.
My gf had gone to have a shower and I was sitting on their sofa with her sister (kitty’s her nickname) watching some tv. Kitty’s an odd combination of girl, she’s very girly and loves pink but at the same time boisterous. She does gymnastics so she’s in very good shape but she is short about 5’2” and she has this perfect little butt that I would get sneaky looks at when I could get away with it. She also has long blonde hair like my gf and I’m not gonna lie she was hot. But back to what happened.
First time
We were sitting there watching tv when she decided to punch my arm. She was trying not to laugh when I looked at her so I gave her a shove. She shoved me back then started trying to push me off the sofa and we broke out into a little wrestling match. We both fell on the floor but kept on wrestling when she sat on top of me and pinned me down. I was thinking of a way to get up and I noticed where she was sitting .. right on my crotch. She was wearing a tiny pair of tight bright pink shorts (I’ll post a link to a pic of her wearing the exact shorts at the bottom), the kind of shorts that barely cover the cheeks. I felt her moving around on me, rubbing against me and I started getting hard. I just couldn’t stop myself, I had always wondered what her ass would feel like in my had and now it was rubbing against me. Suddenly she stopped moving, I knew she could feel my bulge growing under her. My heart was racing, but she didn’t get off or scream like I thought she would she just sat there staring at the wall in front of us. She moved her hips a tiny bit as if to confirm what she felt. I was lying there panicking wondering what she would do. She then put her weight down on me a little more pushing her crotch against my bulge, her hands pushing down on my chest. She started making these tiny movements with her hips as if grinding against me. I was thinking to myself holy crap is this really happening, should I stop this but I was way to turned on. I looked up at her face and she had her eyes closed and she started moving a little more on me, grinding against me. I just sat there in shock of what was happening I mean this was my gf’s sister on top of me but I was so horny and so hard by then. After what felt like hours but was probably seconds I was feeling a little braver. I put my hand on her smooth tanned thigh and waited to she if she’d do anything .. she didn’t, just kept grinding. I moved my hand up her thigh and as I did I noticed a small wet spot forming on the front of her shorts, them being pink it was quite obvious, this just got me going even more.