You make sure the world thinks Peach is normal and has always been like this. You watch her, continuing to grope her giant breasts in the living room (even though she figured she was alone, you realized you must have made a wish about her being super slutty. You'd never seen her masturbate, especially not in the living room.) At first, you thought it was just her transformation, but making her think this was her normal body made her even hornier.
You made another silent wish; you'd be Peach's twin instead of her half brother. You decide to change reality so that it was normal; in this universe, you'd take her old name, Jessica. You remembered another one of your old wishes, that you and Peach would be lovers. You decided to modify this; Peach would be really easy to change mentally and pervert her into a slutty, incest loving twin sister. You wished that instead of having a dad and a step mom, you'd have peach's mom as your mom, and that your dad would be a hot lesbian step mom.
To add in a twist, your step mom would be a hot Asian woman, who was even younger than you and Peach's mom at 25. She had huge breasts, too, but nothing compared to you and Peach. If anything, she was now your former step-mom's trophy wife.
You decided to make her think that incest wasn't normal, and that the sexy thoughts she was having weren't right, but the more you prodded and poked her, the more she'll succumb to her new feelings. You also wished that that her mom, your dad-turned-mom's trophy wife, would find out about you and Peach's secret, but not say anything because it makes her so horny. Eventually, you'd all have an orgy together.
Now, you decided to start your transformation. You willed yourself to become Peach's twin sister with the wand, and immediately felt a shock go through your body. You felt your penis recede into your vagina, and then you felt your hips and ass expand to a monsterous level. You then felt any fat from your body disappear and relocate to your ass, making it even bigger. You moaned as you felt a pair of huge, I-Cup breasts grow on your chest, and your waist pinched in. Your hair grew down to your ass, just like Peach's, and it turned a light brown color just as your face changed to an exact copy of Peach's.