This is what happened: I was at my younger cousins (M18) house hanging out for the weekend but then I had to pee so I went to knock on the door because it was closed, then his older sister/my older cousin who was apparently in the shower answered asking who it was. I said it was me and that I had to use the bathroom and she said ok then come in really quickly so I opened the door and walked in. There bathroom is a glass door not a curtain so I could see her bare ass because he had turned around and I instantly got hard. But I started to pee then I heard her mumble something about it being bigger then she thought and I got really turned on by that. But I finished peeing them flushed the toilet, and washed my hands. But when I turned around she was turned around and had her head sticking out the glass door with the shower off and asked if I could get her a towel so I did. She stepped out of the shower and grabbed the towel so I saw everything and it was amazing. I saw her glance down at my crotch so she must have noticed I was hard but after she grabbed the towel and wrapped it around her I left because my other cousin might have been wondering where I’ve been. But early he texted and asked if we can talk tomorrow at their house and I’m pretty sure she’s home alone tomorrow