Hummingbird (love the nick name

you are in the right place. I found this group about 5 years ago after Mom was diagnosed with Alzheimer's. Needless to say I have stayed around!!
You said your MIL, her mother, and sister all had Alzheimer's disease. You did not say at what age they were diagnosed. Was it Early onset before the age of 60 or late onset? The genetics are different. EOAD (Early onset Alzheimer's Disease) is more likely to be genetic though some do get it without a genetic history. Late onset Alzheimer's has a combination of little understood causes even though it may have a generic component. There are also many other medical conditions and medication side effects that can cause Dementia type symptoms. So you don't know what you are dealing with until he does go to the doctor and gets an accurate diagnosis. It would be a shame if it is something fixable and fear keeps him from finding out.
In my 50's I had a scare myself. Eventually we figured out it was Statin (lipator) that I had started taking for high cholesterol. I went off the medication and my cognitive issues went away. My Grandmother had late onset Alzheimer's. My Mom has late onset Alzheimer's. I had a reaction to a medication. I didn't think anything was wrong with me (typical in cognitive decline) and saw no need in going to the doctor. It was only when I forgot my medication and didn't take it for 10 days that I was clued in. My family and best friend knew... I didn't until I stumbled on the cause. So this gives me a bit of insight into hubby's resistance but also into your need to help him find out the cause.
TC is right in that you need to get the legal paper work together regardless of the outcome. Every individual needs a durable power of attorney, medical directive/medical POA, living will, and will. We never know when this legal paperwork is going to be needed. It's not just for old age or dementia. If there is an accident that leaves us incapacitated it would leave our family able to deal with our affairs. I cannot stress this enough. I thank my Mom every day for leaving me with the legal paperwork that has made my job, taking care of her financial and legal affairs, so much easier.
Men can be very stubborn about going to the doctor and if there is something causing cognitive decline, they do not realize it is happening, and it makes it worse. This is when you have to pull out all the bargaining chips you have. Bargain with him to go get a physical. Don't mention the Alzheimer's to him as a reason but just the fact that he needs a base line physical. If there is another physical problem that you can throw in the mix, then do so. Point out any physical conditions he might have that the doctor could help with. If necessary make him an appointment and tell him you will go with him on X day because you are worried. Pull the "if you love me" card if necessary. Just don't make it about Alzheimer's but about his health.
You can tip off the doctor who can give him a MMSE (Mini Mental Status Exam) as part of his physical. This will give the doctor an idea of his cognitive level. You could also look up MMSE online. You can find a copy of the test without much difficulty. There are 30 question you can ask. A perfect score of 30 is typical of cognitive intact individuals. One point is deducted for each question missed. The lower the score the more cognitive decline there is.
The resistance you see can be from fear because he knows what this disease is about and may be fearful that he does have it. The paranoia, poor judgement, and anger is also part of the disease. Beyond that if there is cognitive decline he may or may not be aware that his brain is misfiring. So he is either responding out of fear because he does have glimpses of something or he may actually have no clue what is happening and therefore doesn't understand your request to see the doctor. That is why it is best to get him there for other reasons and then deal with the dementia after you are there
I bugged Mom for a LONG time to go to the doctor. I was persistent and yep I angered her more times than one... but she finally went to shut me up and prove me wrong. I was not surprised when we received her diagnosis of Alzheimer's. I wish she HAD been right and proved me wrong

But at least we knew and that gave us a path to follow rather than not knowing.
Beyond that, the current medication used for Alzheimer's is more effective the earlier you begin taking them.
It might help to know what symptoms he is showing, what medication he is taking, and what medical condition he might have?
I do hope you can get him to the doctor and find out what is going on. It is better to know than to guess. Hope to hear from you soon and know we are here for any help and support we can give to you
Love, deb