This is something I've often wondered about: Why are siblings' personalities so different? Here's an interesting theory I heard via NPR:

The second theory has a slightly confusing name; it's called the non-shared environment theory, and it essentially argues that though from the outside it appears that we are growing up in the same family as our siblings, in very important ways we really aren't. We are not experiencing the same thing.

"Children grow up in different families because most siblings differ in age, and so the timing with which you go through your family's [major events] is different," says Susan McHale, a researcher at Pennsylvania State University. "You know, a parent loses a job, parents get divorced. If you are three or five years behind your sibling, the experience of a 5-year-old whose parents get divorced is very different from the experience of a 9-year-old or a 10-year-old."