Years ago, I read a book called Runaways about two teenage girls running away from home?

They changed their names, one became a nanny, the other ended up a single mum. One girl ran away because she was jealous of her baby sister, the other ran away from a children's home It's a British author, the book was written in either mid to late seventies or very early eighties. I have no idea who wrote it. Any ideas?