It is a long time since I worked on a unit for people suffering from dementia illnesses, but at that time we always treated the person as a member of a family, because this was critical to helping support people at home. There would have been no problem in making sure that the main carer in the family was informed about appointments.
It is a difficult area, as the staff of the unit will want to respect the patient's dignity and right to retain control over their lives, but in situations such as this some common sense needs to be used as well.
I am sorry that you are dealing with this horrible illness in your family and hope that you are otherwise well-supported.
My OH has PD and is able to deal with his affairs, but there is some frustration on my part in that he does not wish me to be involved in any of his hospital appointments - difficult because there are aspects of his illness that it would be useful for his advisors to know and which I think he does not tell them; but it is right that he should make this choice for himself. It does make me feel irritated at times, even though I understand the reasoning, as I think they should be considering the impact on me and the wider family.