Best Friend's Older Sister

When I was younger I used to hang out at my best friends house. He had a sister that loved to party and would sneak out often. Apparently they somehow came to an arrangement where she would let him grind on her ass in exchange for him helping her sneak out and not telling.

I saw them once she was wearing a big than shirt and just panties and he came up behind her in her room everyone was supposed to be watching a movie in the living room. As I watched he started grinding on her and she just put her hands on her desk and leaned forward. His erection was pushing her panties deep into her asscrack. It didn't take long before he pressed up on her hard and groaned out as he shot his load I confronted him and he told me about their deal. And the next time I came over his sister offered to let me do it too if I didn't tell. So that's how it started we would have her lean up against the wall and push her butt out and grind and grope her. I think she loved it as much as us she always had a huge smile. Especially when she felt us cumming. She would clench her ass on my dick as I pumped my dick into her asscrack and tell me cum for her.