Naive step-sis
A step-sis who thinks she's being tricked into doing something, but really she was just waiting for the best moment to laugh at you.
My step-sis said she didn't know I was going to take her virginity. She was just waiting for the best moment to laugh.
My step-sis cried when I told her I was going to kiss her. She laughed when I did it.
My step-sis acted like she didn't know I was going to sneak into her room. She knew. She was just waiting for the moment to laugh.