Even if she had magic or whatever, Hannah was still your little sister, and there was no way you were going to admit defeat here. You were older, and with that comes experience. You considered your options here. You could try to trick her into changing you back, but that didn't' seem likely to work for long. Still, she is a little girl, if you threatened to tell mom, she'd probably give in. Of course last time she simply turned you back before mom got home, but you didn't' have to tell her Hannah turned you into a girl, just that she wasn't listening to you
"Well, Jessica?" Hannah prodded. "Have you made up your mind yet? We'll have lots of fun playing dress up and with my dolls!"
Coming up with a plan, you replied. "No, I will not play with you, and you're going to turn me back right now!" You stated as authoritatively as your little girl body would allow. "Even if you turn me back before mom comes home, I can still tell her you refused to listen to me." You held up the note mom left. "Mom put me in charge, so you need to do what I say or she'll punish you when she comes home, and I'm sure she'll come up with something worse than whatever you have planned for me."
Hannah giggled at this. "Jessica, you're forgetting, I have magic. I can cast a spell on mom and make her take me out for ice cream instead. But for now, I think you need to be punished a bit. If you won't play dolls with me, I'll make you the size of a doll, then I'll play dolls with you all day!"
Your gamble had failed. Hannah was too caught up in her powers to listen and had even begun to lose respect for mom's authority as a parent. You closed your eyes and braced yourself for the worst when you heard your mother's voice.
"Hannah Harrison! What do you think you're doing?!!" your mother asked from behind Hannah.
"Mom?!!" Hannah exclaimed as she spun around. "How did you get in?!"
"Did you really think you're the only one with magic in this family, young lady?" Mom replied. "Where do you think you got it from?"
Hannah gulped. 'I'm sorry, mommy!" Hannah cried. "I won't do it again!"
"I thought something was strange with how John had been trying to avoid you for the past month, but I never imagined you had discovered your powers already let alone that you were using them like this," mom said, looking very disappointed. She then looked at you. "How many times has she done this to you?"
"This is the second time," you replied. "The first was about a month ago."
"I turned her back!" Hannah said. "I just wanted a little sister to play with for a while, I wasn't going to keep Jessica as Jessica!"
"You still transformed your brother against his will," Mom said.
"I was just playing around," Hannah replied with a pout.
"Hannah, magic isn't a game," Mom explained calmly. "If it's misused, it can be dangerous, especially if you don't know what you're doing."
"But I know what I'm doing!" Hannah insisted. "See!" She waved her hand and you found yourself back in your familiar 13 year old male body. "I wouldn't transform John into Jessica if I couldn't undo it!"
"I have to admit, that's impressive," Mom said. "Most girls your age can't use magic at all yet. But I'm still going to have to punish you for this. You thought you were the only one with magic, and you let it go to your head. That's not acceptable behavior. You also risked making your brother fearful of magic."