Where did the name 'Thirteen Sisters' come from?

This is a question I've been asked sooooo many times! In short, I have three sisters and my husband is one of eleven. So no I don't actually have thirteen sisters, but between sisters and sister-in-laws there are quite a few of us. And to top it all off we all get on pretty well, even though we don't see each other very often. How lucky am I!