You need to test the wand before Mitch arrives, and when he does arrive you would do well to have him in the palm of your hands. Transforming into your step-sister will accomplish both.
As you wish to become your sister, you feel a tingling sensation throughout your body as you shudder uncontrollably. You can feel yourself become a taller, your chest and hips contorting and changing with a sensation defying description. Your hair grows out and changes color as your clothing melts into the outfit of your sister.
You feel a little disoriented, and see your reflection in a nearby mirror. You look just like your step-sister! The wand really does work! You feel and examine your body, and realize that your step-sister must not be wearing underwear and you now aren't wearing any. All you have clinging to your flesh are knee socks, shorts, and a white t-shirt.
You look into the other room. Your step-sister is still there watching television. Mitch didn't live that far away, and would be there soon. It probably isn't a good idea to have two Tiffanys in the house...