"Well, I wish you to be my younger brother." you answer.
"Wait, your NEW younger brother?" your girlfriend asks as she's confused. "Wait the seconds, when did you have a younger brother, hun? I thought you had your big brother and your bratty little sister only."
"Well, yeah I have my big brother who is still in college and that little bitch who is my sister who I don't like, but let's just say that I always wanted to have a good little brother." you explain to your girlfriend as you saw her body is about to change. There wasn't much time to ponder this though as your girlfriends changes had begun. Since you both are the same age of 17, your ideal younger brother would be about 12, and your girlfriend seems to be de-aging and shrinking as well. "W...whatta...?" your girlfriend surprises as her dirty blonde medium hair has turned brown and shortened to a boyish pixie hair. "Honey, you seems getting taller..."
"It's because you're changing, babe. The wish is making you into my brother." you reply as her height shrinks in about 2 inches and a couple inches from her waist and hips.
"Oh no, not my big breasts." your girlfriend sadly replies as her breasts, which were increased from E cups, now deflat on her expanding chest, you feel sorry about her. "Y...you better turn me back when you're done."
"Okay, I will." you reply as you're promise your girlfriend that you turn her back soon, but right now you're going to enjoy your 'little brother' with the summer. Now, her skin gets a light tan and gets so smooth, Finally her face seem to grow younger and a bit feminine, to resemble your mom's and dad's features more than her own. Your girlfriend's clothes has turns into a green tank top, red sport shorts and some white trainers. When her transformation finishes, your girlfriend isn't no longer her... or himself, s/he is now your younger brother that you always wanted. "Ohh... my god what happen?" your transformed girlfriend responds with her... or rather his new young boyish voice.
"Babe are you okay?" you asks as you make sure if s/he was your girlfriend.
"Babe? Dummy big bro, I'm not a baby or your girlfriend, I'm your little brother, you should call me lil bro. Like you always do." your 'little brother'(who was your former girlfriend) replies and laughs, it seems like her mind is also change.
"Oh sorry, lil b...bro. I was just daydreaming." you reply to her/him.
"Heehee, anyway, big bro. What should we do today, should we play a game with each other, or should we watch some movies on the TV, or let's steal some panties my our sister's room?" s/he asks with the boyish attitude.