The younger brother peeked at his older sister having sex with her boyfriend

One fine day I went to my girlfriend's house for the first time. After sitting for a while, my girlfriend's older sister Tsukasa Aoi came home from work and brought her boyfriend with her. They hastily went into the bedroom and did something in there. I went to the bathroom and accidentally saw the door ajar. I looked in and saw her having sex with her boyfriend. She looked so lewd. I was startled and she discovered that I was peeping. After my girlfriend went to the market and the other boyfriend came back, I was surprised when Aoi said she wanted to test the hardness of my cock. I agreed and fucked her a lot with great pleasure.