A friend referred a specific sitter to me, and I can't find the exact name under pet sitters on rover. How do I?

Call Rover Customer Support at [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]. There's no way to search by sitter name. Be sure to ask them why they've never provided that functionality, among other basic selection criteria.

Ask your friend to send you a direct link to the sister's profile.