Kill your mum
From Create Your Own Story
Your mother was one of the legendary dragon said to be one of the only five things in creation that can't be killed.
Well, it turns out the legends were bullshit. You rip her in half with your claws.
And since you are weaker than your brother at this point in the story, she'd have had no chance against him.
I guess it could be that your mother was old as fuck. I mean she was like 6000 years old.