It's the only way she'll live.
Mother: When you were a baby inside of me, you took up all my strength, enough for three children. So when Phoebe came, there wasn't any left for her. That's why she's so sick. But I prayed in that hospital, I prayed like I've never prayed before. And it worked. You see? He was testing me. It's all a test. Every single thing he expects of us, every single thing he expects of us, we have to do. Do you understand? Do you?
Mother: It's the only way she'll live.