Eric - really called Cat - is heavily dependent on his older sister.
Gwendolen knows that. She's made use of Cat so much, even before their parents died, that he can't exist without her - or so he thinks.
But that's what matters, isn't it?
And if the Chrestomanci really gets involved in their plight, Gwendolen won't stop until she rules the world and exploits its power - just like she's exploiting her little brother's nine lives.