Sitter not returning dog

My sibling in SD used a dog-sitting app. The sitter canceled and delayed appointments for my sister to retrieve her dog several times before going silent. The sitter's final message was I'll come to you and drop off (yesterday), which never happened. I am pretty sure my sister will not see her dog again. The dog was very clever at escaping, fwiw. The sitter is located at a farm with no cell service, so I'm reluctant to advise my sister to drop by unannounced. Any advice?

EDIT, UPDATE-She brought the dog after I posted. Her excuse was she needed to help her husband with his new business. I hope nobody else needs to research a similar situation, but if they do, I hope they see the good advice. Thank you.