For the record I hate that song with the red hot intensity of a billion sons, but the sentiment is genuine and applicable to the writing process. I’d wager that family shapes a person more than just about anything else on the planet.
I tend to write characters with at least one sibling that makes them completely insane. This is not a MarySue-in-Progress (I actually have a COUPLE of sibs that make me bonkers), but nothing quite illuminates true character like family. I can be whoever I want to around my friends because what I do and say shapes that reality; I am who I present myself to be.
Some of my friends have never seen me swear or cry. My family’s been there from the beginning. They know that I water work at commercials and that I often swear like a sailor on leave. More importantly, my family knows WHY I do those things and can usually pinpoint the things that can/will set me off.
As a tool for developing character, nothing provides more omph than showing your MC with her family. In my current WIP, MC1 has been abandoned and betrayed by his family; MC2 has outlived all of hers. Needless to say they both have some issues because of it.
So what’s going on with your characters’ families?