How do I get my mother to take the medicine that was prescribed for her?

My sister lives with my 82 y.o. mother and is her primary caregiver. My Mom is on several medications already. Her Dr recently prescribed two more meds: Zoloft for anxiety/depression and another med for her worsening Neuropathy. After two weeks, my Mom still has not filled either prescription. My sister said she doesn't want to takw any more meds. even though they are prescribed. Any suggestions?