You wake up one day and you wonder why your hands are so small, and why your so short, you look in the mirror and are horrified to see your a girl now, your sister walks in your room while you pulled your boxers down

Morning, Onii-chan!

She examines you all over

Yup. Looks like that did the trick. Gotta say though, Onii-chan... I didn't expect you to turn out so cute! Around the same age as a middle schooler, perhaps?

You look at the drink on the table your sister gave you yesterday