We often take my Niece or Nephew with on vacation (we tend to bring one at a time since only one extra fits in the car and they are much better behaved apart from each other.) We took them on vacation with us before we had children, but since our DD was born March 2001, it has been extra helpful having them around.
This year will be the 1st time we are taking one to WDW with us. We invited our 12 year old niece to come along. The trip is a reward for keeping her room clean (something that is very hard for her). She also is paying some of her own expences with allowance money she has saved and a running account she has with us for being a helper with her younger cousin. She will be missing a few days of school, but has aproval from her techers. Some 12 year olds are too cool to go to WDW with their 2 year old cousin and aunt and uncle, but she isn't.
I am very exctied about her coming along, she is very easy going, fun to be around and helpful with her cousin. I am also excited to share WDW with her. She hasn't been since she was my DD age (just under 3) and she is the type of person who "feels the magic". It would be very difficult for her father to plan a family trip to WDW since he is a single father (widower) with 3 children. Since we are DVC members we hope to have a dual family trip with my brother and his kids someday, but for now the trip seems like too much for him.
(BTW her brother went to FLA last year with a friend and spent a few days at WDW, so I don't feel bad about leaving him at home.)