I understand your concerns . There could be many valid reasons why she has not been in contact and you still have three weeks to go .
Have your messages to her been read ?
You can ask THS to contact her as they will have a contact number . Do this ASAP. Either via live chat ( ask to speak to a human ) or by e-mail
Support@trustedhousesitters.com
Hopefully she does get back in touch . You can then insist that if the sit is to go ahead she must share her mobile with you so that you have a backup way to contact her and keep in touch during the sit.
Similar situations have been discussed on the forum with support from THS member services received and a positive outcome Sitter confirmed but unresponsive