Tell her that you and Sir always have the priority to use the washroom. She should ask whether you 2 need to use it before she goes to take bath. Furthermore, she should be the last one to take bath and clean it up after used.
No matter lunch or dinner, she should serve you before she eat. It's a kind of politeness.
not too late you still have to 'live' with her for another year before the contract ends. So voice out all your requirements for her to follow. For my case, my maid has
(a) to serve us the meals and after we finished, then she eats in the kitchen
(b) she has to take bath every night after she finished everything and we do not want her service any more
(c) for your case, you better state that even she finished eveything at night, she should be the last one to take bath when the bathroom is no longer needed by your family. Of course, she should clean it up after use. It is better to restrict the time she uses the bathroom say not more than 20 minutes for bath.