After class, Mateo invited me to his house to study in a group. Because I was curious, I wanted to tour around the house. My feet suddenly stopped in front of the room where vibrant music was emitting. Looking at me, I saw my younger sister practicing an extremely seductive dance, her round buttocks vibrating to the music, making me crazy. Seeing my hardening cock in my pants, she seemed extremely interested and took the initiative to lead me into her room. Her younger sister was in puberty so she had very high sexual needs, she explained. Performing extremely skillful butt shaking and horse riding skills made me extremely happy. In my heart, I silently thanked my best friend for giving me the opportunity to secretly fuck his sister like this.