"Well you're too young," Kelly said with a sly grin. She knew that this would get Katie's goat, and it did. The little girl pouted and stomped.
"I am not too young! I'm old enough to be able to go out with you, sis!" She cried.
"Nope, sorry squirt, you're too little, a girl your age would just drag me down." Kelly was hoping this would give her the in she needed and it did.
"Oh yeah well I wish we were both 12 years old and weren't sisters, we were best friends and shared everything. Then maybe you'd not be such a drag of a sister!"
Kelly beamed, it was exactly what she hoped Katie would wish for, so she wished it that way but wished it so that only katie and she would remember the change. She fully intended to act dumb even as she felt the warm tingle. She added an addendum that their clothes and belongings change with them. Sure enough she felt herself shrinking and saw Katie starting to grow up. The little girl was staring in wonder and maybe horror at what was happening to her as her body filled out.
For kelly the change was different, the world was growing up around her, her clothes shrinking and changing to typical ones worn by a preteen girl. When it stopped, she and katie were the same height and they still looked very similar, they could almost be sisters.