My relatives were important influences in my life. And not just with hand-me-downs. My cousin Andy got me interested in superhero comic books. Great-Uncle Arden got me started rock tumbling (much to my parents' dismay, as it's very noisy) and making something with all the pretty stones I kept picking up.
Large families were beginning to fall out of fashion when I was growing up. I only remember two families larger than my own. Every summer my sisters and I would eagerly wait for our friends, the Keatings and the Fishers, two large families of cousins, to arrive at their neighboring cottages. We played spies, cowboys, or detectives during the days and in the evenings, huge, loud games of Spoons around a long table or Red Light, Green Light across dark lawns while fireflies flickered on and off.
Eight Cousins, Little Women, Little Men, The Happy Hollisters, The Bobbsey Twins - all of these were popular books featuring large families when I was growing up. There were books as well about only children, and those I also enjoyed reading, especially during those times when I wished I was an only child. Times when I wanted to escape from having to listen to older sisters or look after a younger sister. I've noticed books with large families reappearing again. Patricia Wrede's Frontier Magic series is set in an alternate historical period, when large families were common. I used to think large families of 10 or more children were rare, until I started working on my family tree. Ten or twelve children were normal! Not all survived, which meant for sad moments when hitting things like two columns in the 1900 census: "How many children, "How many children living".
Nowadays a family have one child, three, or even five. But that's not always reflected in YA books. In the literature, the standard seems to be one or two children. Often the reader is never aware of how many brothers or sisters a character might have, outside of the main character.
While the 'birth order theory' is no longer accepted, there are some characteristics that first born children share which are different from only children or last born children. I'm the fourth child in a family of five, and I'm quick to spot when a character supposedly from a large family behaves as though he or she was an only child. This could be due to the author's lack of experience with large families. But there are exceptions. Although Harry Potter is famous for being an orphan, he had a cousin. Harry's best friend Ron Wesley is part of a large family. Both Ron and his family are very believable.
Have you come across books with unbelievable families? What are some of your favorites?