Love your avatar shayknee!
I Loved this book, although I read it years ago. Astrid was used to being everyone's caretaker, even her mother's. That was a natural role for her to assume. The irony was that her mother always told her she didn't need anyone. . just herself. And yet she was capable of giving to others what her mom never gave her.
I think she picked Rena because she was exactly as she appeared to be, in it for the money, in it for the free labor, and most of all because she didn't represent another possible disappointment. She was what she was and Astrid immediately detected that. She was nearly grown by that time and just needed to bide her time in the most predictable way.