A cunning (and sinister) plan indeed. They used the sister's mistrust towards them against her. They already knew that she was suspicious of them, however they also knew that she wouldn't attempt to confront them without a catalyst, or a bait. By having the daughter disappear with the baby, they effectively took advantage of a mother's instinct to protect her child, and thus they effectively fractured the Father's relationship with his sister (as well as indirectly his mother). The finisher will likely be whoever's been calling the sister.