As a previous posted mentioned - definitely get her checked for a UTI. My MIL had significant rapid mental decline last December. It turned out to be a UTI which causes dementia type symptoms in older people. We had no idea this was a possibility until We started researching. We dealt with dementia with my FIL and it is difficult to say the least. Thoughts and prayers for your family during this difficult time.
This.
Plus please make sure her b12 levels are checked. Pernicious Anemia/b12 deficiency can cause many of the same symptoms, including delusions, apathy, irrational thought process, increased agitation, anxiety, slurring words, word-finding & memory loss. Believe it or not doctors are not even educated enough in this area. Scary.
Also check for TIA (mini-strokes) - can cause similar issues.
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As for your father - love & patience. Of course with pushing to enact as much help - legal & medical as you can.
My mother had dementia for nearly twenty years. I talked to her directly even in the early years, had to for many reasons (only when necessary - as little as I could not to depress or scare her, always trying to have as much compassion as possible). I got crap back from her of course, but of course survived it. My father was in denial and not a ton of help in the beginning. Was at times a roadblock. When he faced it, it was as though he always knew. Because he probably did. Then he cared for her for many years.
The love & compassion -
As hard as this is on you & your sister (love sent), for a man to fear his beloved slowly changing in front of him is beyond devastating. So it is easy to understand the initial denial and fear.
I found it helped to put things like this -
Let's just check this out to simply cross it off. That's all. (Making it more casual)
Or let's just get this in order, just in case. No big deal, then it is done.
Or it just might simply be this or that, let's just see.
Which it might - never let a doctor diagnose a thing without crossing off many other possibilities like a UTI, b12 deficiency, TIA & other possibilities. Every single one of them.
And some doctors are open to conversations, some are not.
But always worth the try. Because it opens the channels of what they see themselves. Mom's would do nothing without me writing a letter or it coming directly from her. And then someone read her the letter, along the way. Insert big fat sigh on that.
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For others reading -
Other possible symptoms because it is beyond memory -
Gait changes, shuffling, falls without protecting one's face, not swinging/moving arms when walking, poor word use & sudden limited vocabulary, word finding issues, hygiene/makeup changes, apathy, changes in eating - awkward use of utensils & amount of food, pronunciation, a very social person staying home more, changes with driving, irrational thought process (huge flag), paranoia, distrust, delusions & personality changes. Also some positive ones - loosening of boundaries when a loved one was a more rigid personality.
All the best to you. Hoping that it is an underlying condition that is causing the changes.