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Communicate with the sitter yourself. Tell her your daughter usually goes this often and have her try to stick to that schedule. Not all toddlers feel comfortable telling strangers they have to go.
Make sure your daughter has the same routine with the sitter that she does with you. If you used rewards when potty training the sitter should also use those same rewards now that your daughter is having troubles. And have the sitter ask your daughter if she needs to go potty frequently. If that doesn't work have the sitter actually take her to the potty.