My sister knows, but no sure how much. I was home alone and was crossdressing in her clothes on all fours trying to get something out from under her bed. Didn't see her dog in a sexual way at all and knew nothing about dog sex, but he mounted me and because of what I was wearing (her short skirt and thong panties) he was able to get inside my asshole and started fucking me. I was in shock at what was happening and didn't move at first, then I let him keep going because it felt good to me. I had seen him licking himself before and his cock looked about the size of the applicators on my sister's tampons I had used on myself anally a few times. I had seen her horny dog mounting and humping objects before, but he never seemed to do it for very long and looked silly doing it. When he finally stopped and released his tight grip on me, my asshole felt full, but I thought he was finished. I found he wasn't done with me when he stepped off of my back and turned away from me, but was still inside of me somehow. I didn't know about male dogs having a knot on their cock and didn't know why we were stuck together. Shortly after turning, he started coming in me. I hadn't planned on anything like this happening, but couldn't get his cock out of me and couldn't get him to move with me out of her room and into another room to get behind a locked door. I had no choice but to wait there for him to finish hoping he would come back out when he was done. We should have remained undiscovered as my sister normally didn't get home till much later, but that day of all days, she came home early and caught me wearing her clothes without permission and ass to ass knotted by her dog. It was very embarrassing and humiliating. She was mad about me wearing her clothes and thought at first that I had wanted to get fucked and got dressed in her clothes for the purpose and had pulled him up on my back and had forced him to fuck me somehow in what must have been several times together and she had finally caught us. I apologized for wearing her clothes without her permission, told her it wasn't what it looked like, and begged her not to tell anyone. She calmed down as I explained what had happened with her dog mounting me and it was the first time anything like this had happened. She let me know about male dogs having a knot and that it would go down once he had finished. She stayed with me till he was done and had lots of questions I felt like I had no choice but to answer. In spite of her being there, my insides were responding to him inside me, pulsing, and coming and before we separated, I was hit with a good feeling and let a moan slip out, leading to more questions from her. After he pulled out, she made sure that I was okay and had me take her clothes off. I saw him licking his cock and it still looked big. It must have been bigger when it was stuck inside me. The front of her panties I was wearing were soaked and she saw that. She let me know after finding out I was a virgin from the questioning that since this had been my first sexual experience, I had lost my virginity to her male dog and no matter how it started it looked like I had enjoyed it. I was ashamed to admit it, but I had. My sister ended up being okay with my crossdressing and gave me some of her old clothes to wear, so I no longer had to dress in secret. As far as I know, my sister never told anyone about my crossdressing or about what happened with her dog that day. What she doesn't know is that was not the last time it happened as after that I willingly let him fuck me.