Evil Cousins?

Have you ever noticed that cousins make great part-bad guys in books? In "The Voyage of the Dawn Treader" by C.S. Lewis, Eustace is Edmund and Lucy's spoiled cousin, in "The Iron Sceptre" by John White, Mary, the (once again) cousin of Wesley, Lisa, and Kurt messes things up several times on their adventures, and there are probably a bunch more out there.

I think the reason that cousins make great partial bad-guys (although they almost always turn good in the middle/end of the book) is because they're related to the main character(s), but they aren't a sibling so they don't know each other, so they can clash more. When I put cousins in my books I have it that they haven't ever met each other, so they're trying to get to know each other as they're going through their little adventure.

Izori