Mom carefully explained magic to Mel and about the twin factor. She seemed pleased that you might become a girl until mom explained that it would be permanent. It was a comfort to you that she did not want you to be a girl forever, though you were certain she would like you to be her true twin for a while.
"Basically, you and your brother are in a big tug of war." said mom, finishing her explanation. "So long as his physical strength matches your magical power, neither of you will change."
Mel turned pale. "You mean I could change into a boy!?" she asked, startled.
"There is that chance." said Hayley. "Only that has not happened in the past fifty years."
"Right now, you two are just about equal." said mom. "Hayley and I will be teaching Mel magic the day after hers and your birthday, John, so I suggest you start hitting the gym on a regular basis if you want to remain a boy." she paused. "Just don't make a contest out of this, you two."
"Pace yourself." warned Hayley to you and Mel. "Working magic is just like doing heavy exercise, too much of it at once can really injure you."
Mom nodded, as if to make a decision. "To make things fair to everyone, Hayley and I will oversee both of your training schedules so that, hopefully, the balance will be maintained." she said. "However, if you do turn into a girl, John you will be joining Mel in the study of magic."
"Wait, you told me that the balance can be reestablished if Mel becomes a boy, through 'his' dormant magic." you protested. "Why is it not the same with me, through physical exercise?"
"That's a good question, John." said Hayley. "The fact of the matter is, we're not sure. No boy who was effected by the twin factor has ever changed back."
Mel rubbed her arm with her hand, feeling uncomfortable. "So what do we do now?" she asked. "I don't want to feel responsible for changing John into a girl forever."
"Hey, If it happens, it happens." you said, cuddling her again. "I don't like the idea of you turning into my brother, the guilt would kill me too." you placed a brotherly hand onto her shoulder and smiled. "Who knows, maybe I'll be the first witch boy to beat it."
Mel giggled. "Maybe."
"Now all that is left is to explain this to Hannah." said mom. "I was hoping to wait with this until the birthday party." she sighed. "I guess I should have known not to leave you two with a mystery to solve."
You and Harry smiled at that.