You grab the wand again and chant a spell on your step-sister. You chant 'make my sister's but grow until I stop it by clapping my hands together'. As that spell is chanted, the transformation begins.
Your step-sister starts to rise up from the sofa. As you look at her bum, the fat gets put onto it. It grows and grows along with her hips and thighs. Her underwear gets stretched until the wands magic pops her underwear out of existence. Her shorts ride up her thighs until due to the fat, it pretty much becomes a thong. The fabric is like little strings going around her body and the front of the shorts are the same and actually digging into her vagina making her always wet while the bum completely costumes the back of the shorts.
You wait for a little bit longer and then *clap*. The transformation ends. Your step sister now has the plumpest thighs ever and hips to make eve Kim Kardashian jealous. Her butt was now a giant peach, it could even be considered the size of the peach from royal dahl's book games andvtge giant peach.
As a little joke you also made it so your step sister now has the name peach because of her bum. You also were extremely horny and made it so you and your step sister would fuck in secret.