He grins and reaches down to heft you up. You are so bewildered by his statement that you just stare at him.
"I am tired of being the little brother. I get all your hand-me downs, and Mom and Dad keep saying I should be more like you. Well, now you can be the little brother who lives in MY shadow." He looks upstairs. "I found this witch online that sent me the spell. It will be done soon, and then everyone will remember you being like this." He laughs. "I never thought it would work!"
"No!" You twist in his arms, trying to get away. "I'll tell everybody before the spell is done!"
"Oh, really?" Your brother grins again. "What if you can't talk? The spell is still going. I just have to take you back into the basement."
"No!" You keep struggling, and almost pull free. But your brother hurries down the steps. As soon as you are back you can feel yourself start to get smaller again. "No! Please! Pwease!" It is getting harder to talk - like you can't remember how to shape your mouth to make the sounds. Words are still in your head, sorta, but even they are getting fuzzy. You can't get free now - your muscles are weak and you are little more than a baby. But your brother is also smaller - maybe eight now.
You see his eyes widen as he realizes what is going on. "I didn't think it would affect me too!" He starts for the steps and stumbles as his pants fall down. Luckily, he manages to protect you as he falls and twists so he lands on his back with you held to his chest. "oof!" He is maybe six now, and struggles to get up. "No! I have to get out of here before the spell finishes!" He kicks out of his now oversized pants and shoes and scrambles up, only five. You are afraid you might shrink until there is nothing left, but realize that you don't seem to have gotten any younger. You seem to be about 2 years old.
Your brother trips again, this time on the bottom of the tent-like T-shirt. He stumbles, now four, falls to his hands and knees, three, and looks over at you with horrified eyes. Two. You feel strange, and see his features shift slightly. Not quite what he looked like as a two-year-old, more like what you looked like then.
The basement seems to flicker around you, and then you and your brother are in your room. Except that is is now a nursery with a big crib, stuffed animals, and brightly painted walls. You and he are on the floor, dressed in diapers, with some toys. Your mom is sitting in a chair reading. She smiles as she looks over at you. "Guess it's time to put my twins to bed for a nap."