I'm sure you've noticed how authors (and TV sitcom writers) often add a new baby to a family as a way to give them new plotlines. (Weddings, the other big TV plot device, are not, for obvious reasons, as prevalent in children's books.) But did you ever notice how siblings in books are often spaced far apart? Off the top of my head, I can think of the Ramona books (Beezus is 5 years older than Ramona, Roberta almost 10 years younger), Peter and Fudge and eventually Tootsie Hatcher (there's a 5 year age difference between Peter and Fudge and 8 or 9 between Peter and Tootsie) of the Fudge series and, in the books about Anastasia Krupnik, the title character and her little brother Sam (with an age gap of around 10 years). Less well-known, Johanna Hurwitz's Riverside Kids books also feature siblings with a big age gap: Russell Michaels is 5 years older than his sister Elisa, who in turn is 4 or 5 years older than their new baby brother.
The reason, I think, is that the dialogue and action between the two siblings can be a lot funnier this way. A two-year-old can't really star in a chapter book... unless he has a much older sibling who reacts to his antics. There are, of course, books about siblings much closer in age (The Pain and the Great One, another Judy Blume series comes to mind) but I did find it striking that several of the best-loved children's books have brothers and sisters who are pretty far apart, age-wise.
Can you think of other fictional siblings with a big age gap?