I cum on my stepsister's ass while masturbating in her panties.

My shy stepsister gets ready to go out with her boyfriend and leaves her underwear ready to get ready quickly. I enter her room and I find that favorite thong of hers that looks very cute, I smell it and I love that smell of her vagina, it makes me horny, I masturbate with her panties and I fuck her then to keep the secret.