I thought the expensive part of Glacier is retrieval rather than storage?
https://aws.amazon.com/glacier/pricing/

Is your sister doing a lot of file restructuring and reorganizing? I wonder if some moves and sync's are resulting in more retrievals.

Does the AWS Cost Explorer or Billing Details provide any insights? https://console.aws.amazon.com/billi...#/costexplorer My bill shows the breakdown between Glacier storage and S3 - PUT/GET/DEL commands & storage costs