My Sister And I

So this happened a couple of days ago and I just can't keep this only to myself. My sister (18) had her birthday recently. Our parents went away to give her the house space for her and her girls to have a birthday party. I was out for the majority of the day, since I play basketball and decided to just train all day. I'm skinny and tall, while my sister is quite petite, but very beautiful.
So, I came home from practice all sweaty and decided to take a shower. If you're an athelete, you know the post-training boner is a different kind of hard. While I was taking a shower I was thinking about how I'm gonna jerk off when I go back to my room.
Anyway, in the other room my sister was being sprayed with champagne, it's a weird tradition people here do. So she was totally covered in champagne and wanted to ALSO take a shower. While I was showering she came in and saw me in all my glory with my hard cock. We had seen each other naked, so it's not like we panicked, but she got startled and walked right out. After I got out of the shower I put my shorts and only shorts on and went to my room to not disturb the birthday party. My dick was still as hard as a rock but I couldn't get myself to cum, it was just endless stroking. I gave it up and after hours and hours I decided to help my sister clean after everyone had left. I threw out the trash, cleaned everything up and I didn't even notice my cock was so obviously showing through my shorts.
She joked around asking about my boner and I just laughed it off since we always teased each other like that. I don't know what came over me, but I was so horny ans hard that I came over and very gently rubbed her panties through her skirt and that was all it took. She jerked me off through the shorts before I told her we should go upstairs. I lifted her skirt and licked her pussy until her legs were shaking like crazy. She started saying she couldnt handle it anymore and took my dick in her mouth instead and oh my fucking God my dick never felt so good, until I put it inside her pussy. We did it in all positions and she screamed the loudest I have ever heard her scream. When I finally reached orgasm I started pulling it out but she grabbed me and pulled me back in, so I shot all my cum inside of her. A couple of days have passed, I hope she's not pregnant but anytime I see her in the hallway my dick is hard as a rock again.