If I understood it correctly. The trees protect the Earth and the only person who was actively listening to them was Maeve. I'm guessing in return for her listening to them they decided to do something nice by listening to her wishes and giving her back her sister. What would be more interesting is knowing what happened to the sister in the first place. Would be pretty smurfing weird if she'd died of a disease or something.

Or I could be utterly wrong on the reasons why they brought the sister back.