My Dutch grandmother was one of thirteen, and my Dutch grandfather was one of eleven, so I have to wonder sometimes too...
Update: Please, people, this is not intended as a joke about Hispanics or Latin culture. I have great respect for both, or I wouldn't have chosen to make one of the two protagonists in my series Hispanic. Zia happens to have a large family. That's less common today than it used to be, but lots of people have large families — my father and grandfather included — and that's not specific to any one culture or ethnicity, and there's nothing wrong with it. The joke here is entirely ew, your parents got it on a lot. Don't read into this something that definitely isn't there.
Also, the size of her family is important for things that will happen later: The story definitely wouldn't work right if Zia's an only child. Those of you who read Caity's World know that I plan the hell out of my stories, and there are good reasons for things, comedy or not.