First of all, let me express my condolences on the death of your mother. I'm sure you miss her terribly. As for your dad, his rapid decline as your mother became ill may have just seemed rapid because she was taking such good care of him. When she was no longer around to provide his care, his Alzheimer's became more evident. My mother is in the late stages of Alzheimer's and my dad died of dementia. Both of them were on meds in the beginning, but they didn't help at all. It may be true what they are telling your sister. My mom's had it for ten years now and is slowly but steadily getting worse. If your dad progresses rapidly it would be a blessing. You wouldn't want him to live like my mom, helpless, in diapers, unable to talk, walk, or even feed herself. Nursing homes for the most part, suck. I empathize with you as do a million other family members out there. God bless you, your sister, and your dad.
Alzheimer's disease is a chronic degenerative disease with no known cure
Despite intensive research the exact cause is still not clear, but new treatments to slow the course of the disease may be on the horizon
Today, there a 2 types of medications that have been shown to slow the clinical course of Alzheimer's. No medication has been shown to stop or reverse the disease process
The acetylcholinsterase inhibitors such as Donepazil, Galantamine and Rivastigmine can slow the cognitive symptoms in many patients(such as memory etc) but only by several months. Although most studies did not look at the effect on caregivers, a study with Rivastigmine showed a significant benefit to caregiver stress and ability to cope. These drugs are now fairly standard of care for Alzheimers dementia.
Another drug recently approved for more moderate to severe Alzheimer is Memantine, which works in a different way to the above drugs but has also been shown to slow the clinical course of the disease. All these drugs are approved by the FDA for Alzheimers(Federal Drugs Administration).
Hallucinations and rapid fluctuations are a feature of Lewy Body dementia - I am not sure if this is a diagnostic consideration in your father without seeing him myself. It is similar in ways to Alzheimers and the same treatments above are effective also
Clinical trials are essential to our advancement in treatment of diseases like Alzhemiers, and this is how the above drugs were shown to be effective. I do not know the details of the trial, but you should discuss your concerns with his doctors, and whether any of the above drugs might be indicated. The benefit from these drugs is limited - that is why we are still looking for more effective treatments.
I know he won't be "cured" by conventional meds...I just want my Dad to function as well as he can for as long as he can. I'm only asking what you would do in a similar situation, so I can show this to my sister as just an objective take on what is happening.
Thanks again.