Best fun I ever had and secretly

When I was almost 15, I was visiting with my cousins for the summer in Norway. The day before I was going to leave back home, while she and I were still in the sauna, right after everyone else had gone to the house, my chubby 9 year old girl cousin admitted having had sex with her brother, my 13 year old boy cousin. I was kind of shocked that she'd tell me but I got hard as she was telling me about it. I had all sorts of thoughts going through my head. I knew it would have been a huge risk for me to take to do anything so got up to look out the little window on the sauna door to see if anyone was around. Even though I had a towel on, my erection was obvious. She and eye made eye contact for a few seconds before she asked to see my penis. I pulled my towel up, exposing my erection to her. She stood up immediately and removed her towel. She turned around and draped herself over the bench and parted her butt cheeks with both hand. She made it clear that I could only buy it into her butt hole. I did not hesitate. I gently slipped my penis into her butt hole and she totally tolerated my penetrating her. I humped into the very warm and firm grip of her anus. I swear I came in like 30 seconds and pulled out. She stood up, wrapped the towel around herself. She pressed a finger to her lips, making that "Shhh" sound and out the door she went. It took me a bit to get her myself and I followed her to the house. OMG I was so scared she:d say something or tell her brother or something. Dinner was so awkward for me. I left the next day. She and I hugged like nothing had happened. A decade later I heard she got married from my dad. I have not visited Norway since that time.