My father-in-law is making serious accusations regarding family members. These accusations are from left-field and are targeted to in-laws that he resents whom he feels caused his divorce over 3 decades ago. Is it possible he may in the early onset of Alzheimer's or is depressed?
Obsession is early dementia, take careful notes of date time and things said and leave out opinions and inferences.
As Psteg suggests, get him to dr and request full medical mental health work up. As I've suggested before, document all your observations, crazy things he says or believes and send them in advance to the dr so he can direct his questions and exam to uncover any underlying issues. Dr may suggest neurologist exam as well.
In my moms case they prescribed antidepressants, aricept, namenda, anti anxiety but the drugs didn't improve her and mom eventually refused to take them saying "I don't feel myself". She stopped altogether and I have to say as long as she is engaged, I can call or visit, and she goes for walks or is out and about more, she functions very well and a lot of the paranoia, craziness, confusion dissipates or happens infrequently.
My point, is isolation, loss of friends, family, interaction with the outside world can exascerbate dementia. I have learned not to try to set the story straight, I hear her out, and take a deep breath and don't react to her accusations.
Good luck.
It is entirely possible that your FIL's behavior indicates the beginning of dementia. It is more likely that the dementia is causing the obsession with his divorce than the other way around. He could also be depressed or have other mental/emotional problems.
Whatever the cause, it would be ideal for him to be evaluated by a doctor. Do you have the kind of relationship with him that you could bring that about?
Meanwhile, if you are a targeted in-law, consider the source. You don't need to defend yourself or dwell on the accusations.
My advice would be to find a good geriatrician i find them much better than GPs and have much more experience with this.
I realised mum had something worse when she was continually doing dangerous things and not understanding just how dangerous this was she had lost all sense of reasoning.
But get an expert to diagnose as it could be "deep depression". I dont think depression causes dementia BUT if you become inactive and dont get some excercise and spend too much time doing nothing your brain will deteriorate. Dementia can cause depression in some my mum seems less depressed now but when she was told at first she had some depression then came denial and here we are............. TOTAL DENIAL
I hope its just depression but i think you will know yourself that somethings not right.
Also be prepared to have "dirty secrets revealed" as to family my mum came out with alot of family stuff that we found out to be true that shed never have told us before?