This is a great movie, definitely worth a watch. I don't agree with a few of the previous reviewers on here who possibly weren't paying attention..
**spoiler alert**
In the film the 'Aunt' clearly states that the reasoning behind what she did was due to the fact that her and her husband were spreading the word of God when they lost their son to cancer. They blamed God and lost faith so they abducted children so that their families would turn against God too. Seemed pretty clear to me :)