My mom is in the hospital. If I have to go there how can I get a sitter/walker to replace me for my clients?

In addition to contacting Rover support, I'd use Rover messaging to communicate with clients and let them know your tentative plans, in case the client already met/considered an alternate rover sitter as second choice.

Call Rover support immediately @ [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team], they will attempt to re-home your clients