Rovers insurance does not cover if the dog bites to the sitters. As far as you being held responsible, NO you would NOT be responsible to cover medical bills either.
As long as you have been open and honest about your dogs unpredictability and made sure the sitter was informed and aware of your dogs situation you are not responsible if your dog does bite the sitter.
Being that you are paying the sitter, she is therefore considered a "professional". As a professional pet sitter it is her responsibility to know which dogs she can handle and which ones she can not. She should not accept the dog if she is not confident that she knows how to read the behavior of the dogs and knows how to deal with aggressive pets. When she agrees to watch your dog, she is agreeing to accept responsibility for the pet and his/her actions while your pet is in her care.
So it does not become, "a he said she said" situation. I would recommend (if you have not done so already) make sure that you are using your Rover numbers, this way the conversation about your concerns is documented. You, the sitter, and Rover will all then have a copy of the conversation.
(working in an animal hospital for 20 years, one of the first things we are taught, is if it is not written down it did not happen. So just make sure everything is written down.)
Best of Luck!!