Maybe she feels safe and secure in her position in the family so she doesn't feel she needs to compete with her brother for your attention.
Or... she's paying her brother to keep you busy so she can do what she wants without all that icky family stuff.
Or... she has a wardrobe in her room connected to Narnia.
I'd guess as long as you make yourself available when/if she decides she needs it, you should be cool.
