I am thinking that my mother is in middle stage dementia. I believe the next stage coming, but how can I be sure?

I Googled and found this

https://www.thegardens.com/understanding-dementia-stages

Stages can overlapse too. If you go to You tube, put in the search "Alzheimers video". There is one thatvisca cartoon with a yellow brain. I found this was a good one explaining how the brain is effected. My Mom did not have ALZ but she pretty much followed this video in her Dementia journey.

"Her face is plain" may mean that she is showing flatness of affect. That means that she isn't showing emotions, such as smiling as she used to do when something was pleasurable. This look might also be described as "a spacey look," like she's not exactly with you in the moment.

You are asking us what the nurse means by "your mother's face is plain"?
I think it would be best to ask the nurse herself what she means, because I have utterly no idea what that means.

As to the stage, they are not predictable. You can get a good idea of what the stages ARE by looking up "staging dementia" online. But your Mom is unlikely to fall neatly into some categorical slot.
The one thing you need to know about dementia is that it is individual for each person, and progresses on an individual timeline, different for each person. Daily you see what you see, and that--to be honest--is where you are for that day, that moment.

As the weeks and months pile up and accumulate you will see that there are more and more problems, and less and less to be done about them. Eventually you may approach a time when you cannot do hands on 24/7 care of your Mom without giving up your entire life to it. That is a time of decision making for your own life.

Your mom's doctor is the best one to discuss her individual case with. He/she knows your mom holistically. That is to say knows her history, her general health, her attitude. Can give you the best diagnosis for now and prognosis for the future. But even for medical there is a lot of guesswork involved. We are each as unique as our own thumbprint.

I wish you good luck.

Part of dementia or Alzheimers is they have trouble swallowing so find easier things for her to eat Like mashed Potatoes . Face being Plain Means she Is showing No emotion . Often times they will space Out and be in some other reality . Yes her thinking of you as her sister and Not her daughter is common . There were times My Dad thought I was my Mother . Join a caregiver support group the Alzheimers association Has a 6 week course the Savvy caregiver - it is free and they will offer support groups with a therapist .

Your mother should be having her meats pureed or cut up in very tiny pieces if she's having trouble eating them. That means that she's having swallowing issues which could lead to aspiration pneumonia and even death. So please have her doctor order a swallowing test to be done, as she may have to have all of her foods pureed and perhaps all her medications either crushed or in liquid form(if available)and her drinks thickened as well.
Someone with dementia will only continue to get worse and it sounds like your mother is pretty far along in hers with her swallowing issues and not knowing who you are.
The best thing you can do now is to get your ducks in a row as far as who will take care of your mother when you are no longer able. I hope you're thinking that far in advance.
It doesn't really matter what stage a loved one is in as each different dementia(and there are 100's)has different life expectancies. So unless you know exactly what type of dementia your mother has, you will be left guessing.
And just as an example, vascular dementia has a life expectancy of just 5 years, and Lewy Body dementia 5-7 years, while Alzheimer's can go on for 20+ years.
I wish you well as you take this journey with your mother.

What difference does it really make what stage she's in? You deal with problems as they arise one day at a time, and make a plan B for what you will do when you can't provide the care she needs any more.
As for the difficulty eating - choking on meat is scary as heck and often one of the first signs of swallowing dysfunction, she probably should only be given minced moist meats now and checked for dysphagia.

I suppose that the nurse is telling you that she doesn’t see any emotions or expressions on her face.

I am so sorry that you are going through this struggle with your mom.

Sending hugs and support your way.