My mother is in early stages of Alzheimer's/dementia. I have several siblings, two of whom are very dictatorial in their speech when trying to get her to do things that are necessary (i.e. reminders to take med; to wear incontinence products when going out). The more dictatorial they are, the more she is embarassed and of course stubborn.
Often just the "tone" of voice is all that matters. That is all you "hear".
It might also be a good idea to get everyone to a Support Group or find out what videos the Alzheimer's Association has on line and they can see what works. The Alzheimer's Association also has a number you can call and talk to someone about frustrations and problems you may be having. They may give you some good suggestions as well as tell you what videos to view. They may also have local programs on the various stages and how to cope with problems that arise.
I'd suggest you suggest to the sergeant major siblings that they listen to themselves. And then think: if an outsider spoke to your mother as they just have, how would they react?
Maybe there is a social worker or other third party who can help your siblings understand that they need to change their approach? A social worker visits from time to time and her input has been invaluable.
Perhaps you can talk to the dictatorial sisters. Good luck.
Also, her reasoning is going, so that's another reason I don't go into long explanations. This frustrates me, because I know that I would like more explanation on things.
One time she said, "You don't have to yell at me!" That was just one time, though. All the other times it's the case that I have to repeat what I've said, sometimes multiple times.
I might also get a consult with an attorney, to make sure that I was on sound legal ground.
For me, honesty is a good thing and I would have no problem telling them to stay away if they can't act right.