ah, read all the posts Meg! Yes, I think you're on the same train of thought I was thinking. I'll leave the post in case it tickles a different thought in your head.
writing in 3rd person then as i assume the story is about the 15 year old or is it about the family left behind? If you are primarily in the 15 yr-olds head in first you don't have to speculate. It might be interesting to write a lot of flashbacks to previous situations that show your tight knit family, good relationships, etc and carefully select those that have a tiny hint of what eventually causes the rift. In other words, while the 15 yr-old is running... use flashback scenes to reveal the family.
That way the reader gets to figure out the reason for themselves, and you as well.
Just one idea...
On the other hand you can reverse it. If your focus is on the family, then use flashbacks to reveal what is going to go ka-blooey in the 15 yr-old's life.
This is an old post, so I assume you've already written it. Challenge you to post!?! :-)