Jon's pleading didn't stop his mother from uttering her wish. A wish that the now-teenage mom had no belief could happen.
"I wish I was your little sister so I could torment you," she said.
The wishing stone glowed, granting her wish. But it wasn't granted the way Jon expected.
Jon's mother had already wished to be a teenager, so wishing to be Jon's little sister would be a contradiction of that wish. But ... she could still remain the same age as she was now and still be considered Jon's little (or rather younger) sister if Jon was older.
And that's exactly what happened.
Within seconds, Jon aged by ten years, becoming 26 years old.
