My boyfriend confused me with my stepsister

I went out on business, and my stepsister stayed at home with my boyfriend. I've noticed for a long time that she looks at my boyfriend. And I was right. As soon as I left, she came into his room. He did not distinguish us well and confused us this time, but she did not miss this opportunity and straddled him. I went into the room when he had already finished in it. She was amused that it happened, but my boyfriend was not laughing. It really excited me and I wanted to join them, but that's another story.