It's possible that the main characters include not only the foster girls but also the people who influence their lives the most. This could be their biological parents who may or may not be part of the story in some way. Also, there could be other foster children in the same home who have an impact on the main foster girls' experiences and development.
Perhaps the main characters are a group of foster girls with different personalities and backgrounds. There might also be some important figures in their foster families like kind foster mothers or strict foster fathers. And there could be social workers or teachers who play significant roles in the girls' lives in the story.
I don't know as I haven't read the book. It could be the foster girls themselves.