Sarah is the little
The day of the massacre of adults in Gatlin, she was with her mother at home sick. That's where she developed her second sight while every adult, including her mother and father, were massacred. She has to witness the order of the death cult. She belongs to the few children who didn't join the cult, but Isaac Chroner tolerated her and Job because of her second sight.
When Vicky and Burt come to Gatlin, they join forces with them to defeat the cult and succeed. After that they all go away from Gatlin and report what happened to the authorities. It is implied she and her brother will be adopted by Vicky and Burt after that.