It was then that I felt something different. In my fogged state it took a minute to realize what it was. My sister's bikini bottoms had come untied and were slipping down her leg. We walked maybe 6 more steps and they fell all the way off.
We both stopped walking and paused with realization at what had happened. She whispered back to me.
"Oops, sorry. I guess this is a problem, little brother," she said in a giggly laugh.
Now that her bikini bottoms were totally gone her ass really spread around my cock. The feeling increased ten fold as each step she took caused her ass to pull my cock up and down. I let out a moan as I felt her naked ass against me. She whispered back, "We need to get back to the room and fix this problem before things get messy."
She had no idea how messy things were going to get very soon if we did not get out of this thing. I could feel the blood leaving my brain and all I wanted was to cum hard and long. I was close to the point of not caring who I was with or who was around. I just wanted to cum. Her bare ass was so deep and tight that I could feel her cheeks pulling on my boxers as we walked. As she took one more step her ass pulled on my cock and I felt my cock spring out of the fly in my boxers.
"Oh shit..." was all I could manage to say as my cock pressed against her wet ass.
"I know I can feel it" she said in dreamily wispy voice.
"We have to get back to the room now and get out of the costume," she said. This time there was no laugh just a small quiver in her voice.
I only thought I was turned on before. Now I could feel her bare skin and the sweat from our bodies running between the crack of her ass. The whole inside of the costume smelled of sweat and there was no denying there was the smell of wet pussy.
The smell of her sex was more intoxicating than anything I could have ever imagined. I was getting more and more turned on. I knew in my brain that we had to get out of this soon, but all thought about consequences had totally left me. I was completely and totally lost in the moment.
My heart was racing and I could tell she was also starting to pant a little. Her breathing was short and fast and with each step she would let a little moan out. Getting back to her room was a nearly impossible task. We were way outside on the deck and it took 10 minutes for us to get that far. I knew it was going to be a slow walk back inside and to her room.
Slowly we walked back inside her house. Each step seemed to take and eternity. With each forward step my rock hard cock was slipping up and down her and I swear she was pushing her ass against me with each step. The temperature in the costume was rising and I was not sure it was all from the heat.
I whispered, "If we don't get back soon I am going to lose it... We have to try to go faster".
We picked up the pace and started taking bigger steps. This was a big mistake. With her first big step forward we got out of sync and she fell over toward the floor face first. I tried to lean back to keep us upright. Her falling forward and me falling back was just enough gap that my cock slipped from between her ass cheeks. As she caught herself from falling and pulled up, I felt a warm wetness engulf my hard throbbing cock.
She let out a moan and her whole body stiffened as a small quake rippled through her body. My mind was in a fog. I tried to pull back but there was no room. Her pussy squeezed my cock like a vice and her breathing quickened. Neither of us moved or said anything for several seconds. Soon her breathing slowed down but the warm wetness was still engulfing my hard cock.