"...Police inspectors were unanable to disclose how the girls’ family address was known. However, dozens of potential avenues of attack exist..."
just guessing here... exif geotag data?
I don't think that data makes it through when Facebook resizes the picture. At least, I wasn't able to extract it from any of the photos at the top of my feed just now, including pictures from a couple of smartphones.
Good old fashioned social engineering? The attackers already knew where she was going to be from other location based apps or just investigating the address of the grandmother. Also, the attackers could have access to the daughter and she is just pretending not to know anything to help the grandmother file an insurance claim.
She's from a pretty small town (population ~2000 according to Wikipedia). If she listed her hometown on facebook, her last name, a phone book, and a photo of her home would be more than enough.