If she's going to act like a selfish baby, you may as well make her body match, no? You point the wand at her and try to make her younger.
She doesn't change much at first. She continues chatting on the phone, but then pauses. She looks like she's concerned about something, and you can probably guess why. Her breasts look smaller, her hips have lessened in size, and her clothes just generally look looser.
She continues to lose her more bouncy assets as she drops the phone, and she starts to lose size in other places. She's becoming shorter, her clothes rapidly becoming looser, and her hair gradually getting shorter. She stands up suddenly, her shorts and sock slipping, and her shirt drooping. The sudden movement catches you off guard, but she's only looking at herself. You feel compelled to keep regressing her.
Your step-sister panics as she reaches down to pull up her now fallen shorts, all the while becoming smaller and younger. She looks like she belongs in elementary school as she starts cursing and looking around her.
She looks right at you! Her eyes are wide as she cries out "you!" and starts to run at you, her extremely loose clothing flopping as she goes. Startled, you let out a burst of magic through the wand that's actually visible and creates a sound. With a flash of light that glides across the room, she dwindles to a baby mid-stride, and falls in a pile of clothes.
She rolls on her back and cries, her limbs flailing, and her feet kicking up her shirt. The hair on her head is but peach fuzz now, and she's making quite a ruckus...