"I wish ..." she began, but then stopped when she felt weird all of a sudden.
She started to grow older. 6 years old, 7 years old, 8, 9, 10. And after a few more seconds, she was a 16 year old again.
"Like, what happened?" she said out loud.
What she didn't realize was that she went back to being a teenager because of her wish. She had said I wish I was your little sister so I could torment you. She was no longer tormenting Jon, so she ceased being his little sister.
Jon was relieved that his mom wasn't his little sister anymore. But she was still far too young to be his mother. And she still had the wishing stone.