Forced to cum inside sister

This was the most embarrassing Moment of my life. This is a real Story and i was a virgin Then. So When i was Younger my sister and me went very often to the water Park in my town. Sometimes on our own. In the Summer Holidays it was best to go there under the week in the morning because in the morning there is basicly No one around. So we did it AS usally. We used to take one slide together while using only one of the tires. Well since the Last year we both Had grown in size and weight. But we did not took that into Account when we decided to Take the slide together again.i also used my old bathing clothes that wasnt Really fitting anymore. I didnt Had one of these boxershorts but one of these Slips. It was just enough to cover Everything and i knew i had to buy a bigger one soon. My Sister on the other Hand looked hot this year and had bought a new one but hers wasnt really fitting either. She bought hers because it looked Sexy but she complaint all the time that her Panty got loose because the sides ribbons didnt hold correctly. Anyway we took the Slide. She was sitting on top of me and thanks to the tire her ass basicly sunk into my hips. I mean it wasnt a Problem...not yet... Her ass was very good feeling down there. I was immediatly shocked when i realized that it aroused me a little bit. So i concentrated on the slide. It Had two ups and Downs but we didnt even reached the second one. We Slided down very slowly and got stucked at the First down. We tryed to get ourself back upwards but it was very tight and we actually couldnt even seperate because my sister was just barely able to get her hips over the tire. We screamed like 15 Minutes for Help until we realized it would Take Other customers who want to use the Slide to find us. So we were trapped. The worst Happened now...my sister had to pee. She could not hold it any longer. So i agreed that she could let it flow. Big mistake. The Moment the warm piss reached my dick and run down my balls i got unwillingly aroused and my Penis got hard. But this just wasnt it. Since my Slip was so small He grew out of it between the legs of my sister. At first my sister just stopped peeing without saying anything but then she continued. Afterwards she looked at me and we Had a small embarrassing chat that im sorry and that we have to deal with this now. Of course she was disgust and in denial that this is happening now. Her ass wiggeling around while complaining didnt helped either. She opened her legs AS far AS she could in the tire to not touch my Penis but it was No use. He was lined up to her pussy lips and everytime she moved He was moving up and down in between them. I told her to stop to move around because i might would have an accident...big mistake telling her this. She got scared and screamed for Help and tryed to raise her hips over my Penis so it would lay behind her back and Not between her legs. I tryed to seperate of course too. So she made a Bridge and told me to grap my dick and move him immediatly! I tryed to move him Backwards from her left pussy lip down her leg but then she slipped with her hand and fell back onto me... I knew the Moment Everything was warm and squishy around my dick that this just had happened. My sister moant very loud and Then we looked in each others face in shock. While she fell my dick Just moved her loose panty to the side and lined up with her Vagina, ready to prope she forced my dick inside her.
Every move was torture now. Every wiggle made me get to the edge. She went furious and tryed to get him Out doing Bridges while i screamed to stop now!!! She was riding me like a Pro. And i couldnt hold it any longer. Before i reached climax i grapped her at her hips and pushed her down. Hugging her very strongly. She stopped...i told her...i was close to orgasm. My dick was already throbbing, Not pumping but He twitched every Minute here and Then.
We both were close to tears when she told me, she was close to come too...we screamed again for help in our Desperate Situation when i felt it....she grapped my hands that were around her hips to reduce the movement and Then she moant in agony and under tears before her inner walls started slowly to contract and squeeze my dick. Her moans were very silent and long while the contractions got faster and more often...i was in emotional pain because i knew she had crossed the road and there was No way to stop the orgasm now for her. She even pulled her legs back together like this would help now. Me on the other Hand was completly suprised how Strong her vulva was squeezing and every time it brought me closer to the point of No return too. Every contraction made me twitch... And Then she said sorry....one word...just sorry...and moant very heavily and strong and very very long, i have never heard her making that sound again, she grapped my hands very heavily and the contracts stopped for a second her Insides got super tight now, i had to moan too now and then the tightness exploaded in even stronger contractions but not slowly like before - it was like two contractions per second. My brain was exploding too i have never felt this before and i tryed to hold on but after nearly 20 contractions i was orgasming too i started to pulsate and blow my load deep inside her. I grapped her Hands now too. We both were in tears while moaning and cumming. We even synchroniced - my pumping was welcomed By her contractions. I was done after twitching 10 times but she continued. I think i counted 37 contractions and we shared our Finish from 20-30...
Afterwards i finally swelled down but it was already to late. We didnt talked for the whole rest of the stay. We were found 30 Minutes later. And immediatly left. We agreed to never talk about this anymore and we have never been together in the water Park again. Every time someone Just meantions a waterpark we both feel Bad... Till this day i regret everything. The realationship with her wasnt the same anymore afterwards.