POV: you've been making fun of me because you caught me in my granny panties. As revenge i sneak into your room and take a picture of YOU in YOUR tighty whities.
you are terribly embarrassed and try to cover yourself, but i keep making fun of your underwear and talking about how i am going to show the picture to everybody.
you get down on your knees and beg me not to, which makes me laugh even more. you look so pathetic on your knees in your tighty-whities. i tell you i might not show people, if you agree to be my slave. i tell you to crawl over and kiss my feet. While you do, i order you to apologize for spying on me in my underwear.
i then tell you to apologize to my panties now.... and i drop my pants, revealing my purple granny panties, and tell you to kiss my ass in them.
i make you say how sexy they are, how much you love them, how great i look in them, etc... i tell you that your new name is panty boy and you are going to be my personal undies slave. you are going to handwash my underwear, fold them, i might even make you wear my panties! As you kiss my butt some more, i muse on other ways to humiliate you (Spanking you, making you rub my feet etc...).
eventually i turn around, and laugh at how silly you still look in your tighty-whities.
i reach out and start to tickle you, laughing at how much you squirm and loving that i am your master now. i tease you with tickle talk ("tickle tickle, coochie coo"). i don't stop until you tell me that your name is panty boy.
i then tell you to crawl out and get me a beverage in your underwear. Panty boy doesn't get to wear clothes unless master says so!

Sit back, relax, and let yourself be swept away into a world of pure pleasure and unrestrained excitement. Welcome to StepSis Makes You Her Panty Boy.