Mary-
I think it is quite common for the younger of two sibs close in age to "play dumb" to some extent. It's as if the younger kids feels she can't compete, and simply gives up and goes in another direction.
We have had many discussions over the years about ALL the kids in the family being PG. I keep telling them that they have individual strengths and weaknesses, but they are all in the same intellectual ballpark. I think they have internalized this pretty well.
My seven year old is more apt to march to her own beat and she is far less linear than her next oldest sibling. Sometimes it is difficult to get her motivated to do work that is not of her own choosing. She will spend hours drawing and writing creative stories, but often balks when I assign her writing. I try to go with the flow as much as possible, but I do insist on a certain minimum in the areas where she is less inclined to work. We are homeschoolers, so this is a bit easier for us to play around with things and see what works.
If your daughter is enjoying C. S. Lewis, you might look for the "Lion's Paw" website. It's rather simple, but my daughter loves it. Maybe she is looking for some feeling of control over what she is learning, or how she is learning it.
I'm not sure if I am being very articulate this early! I hope I have said something helpful.
best wishes-
