Hello, This is Dr. Klarida Papaqako answering your question. Fluids are a very essential part of keeping our body energetic and active, since around 60% of our body is made from fluid. This amount gets lower with age, lower when the metabolism is slower and when the physical activity is low. When the body needs fluids, the brain activates thirst. so unless a person have problems with swallowing or problems with the center of brain that control thirst, the body of every person should be able to take the amount of fluids that his body needs. If I was her caring doctor, would not worry about this in your mother-in-law. I would also make sure she is taking liquid foods, that she likes to eat, with enough proteins; when she does not sleep encourage her to be active and entertained for as much as its possible, and make sure she is not in a depression state; multivitamins may help her with appetite; sometimes having a bottle of water, or a juice she used to like, near her bed and on her reach, may stimulate her to drink more. Hope this information helps. Regards.