When would a doctor declare our father (91) to be not of sound mind?

Anytime an elder is still able to live alone, it's always best to practice precautions to keep bad stuff from happening, including theft, especially irreplaceable items. Theft can happen with or without violence. I don't condone any kind of elder abuse, not even theft. It's been said that things can be replaced, but remember, somethings cannot be replaced. Be glad it was just theft and not something far worse like what was on our local news.
Being elderly and hiring help can't be risky business. The lady helping this elderly woman ended up finding the checkbook and forging a check, stealing thousands of dollars from her elderly employer. I don't recall how it was found out, but the employee was caught and punished with a judgment of full restitution. Sadly, the ex-employee disguised herself as a package delivery worker and got her former employer to open the door. When she did, she ended up killing the elderly woman with fatal blows to the head. Little did she know, the elderly woman's foreign exchange student was in the back of the house. When I saw the story on the news, I was almost yelling and couldn't help but ask the elderly woman to myself why she ever opened the door, because had she not answered the door she would've still been alive.

It's always a good idea to practice safety precautions when you're young. That way, when you're older and more vulnerable, you'll be much harder to get to and take advantage of. If no one can get to you, they'll give up and move on. It's always a good idea to be careful who you open your door too. I don't even answer the door to unannounced company, nor will I even acknowledge them. One trick I learned years ago is that if you do go to the door, don't open it if someone you don't know is out there, They can bust you in the head and rob you blind. By time you awaken, the thief is gone with everything of value, maybe never to be seen again unless you happen to be very lucky and happen to get your stuff back. No matter your age, never answer the door to anyone you don't know, especially anyone you're not expecting ahead of time. With today's technology, there's no excuse not to contact someone you intend to visit to arrange that visit at the convenience of you both. I rely solely on technology to arrange my visits. With very few exceptions, I won't let strangers in or even acknowledge them at the door. As a friend of mine pointed out, if anyone ever broke in, remember the whole host of weapons in the kitchen! Anything you have that can be used as a weapon is in your favor if someone breaks in, but women in particular should remember to first check the kitchen because they know where all the knives are along with anything else they can use for personal self-defense. Anyone of any age has an advantage in protecting themselves and their homes and they can prevent entry to anyone who doesn't belong in their homes. Even for something as simple as a theft can be prevented if you know what to do because prevention is key.
Let this be a lesson learned. Now is the time to take your experience and equip yourself and any elders who still live alone.

my Dad also accuses the transitional carers he had come in when he came home from hospital after a fall, of stealing his watch and some DVDs.

Im inclined to believe him, because the watch and videos are missing and Dad was a sitting duck,(as a poster said above) for this kind of stealing as he was very vunerable and had limited mobility at the time.
It is sickening to think that some so called carers prey on helpless old people like this but the human being never disappoints in what they are capable of,when they think they can get away with it.
It is an issue for us all, who may also need carers one day.
It should probably always be reported, because those in charge may eventually be able to pinpoint the particular carer involved.

Your father's physician should give him the standardized age test, e.g. "what year is it,"et al.

Hate to say it but your problem sounds like the sibling. Might be easier to deal with that, find out from sibling why dad thinks your taking stuff. Just saying.

That's sooner someone starts speaking up to the doctor about observations, the better but that's only if you can actually get in with your dad to his doctors appointment because some patients won't allow it. I strongly agree how hard it can be to get someone declared incompetent but it's definitely not impossible if certain things are obvious. However, sometimes if a doctor starts noticing something the patient may voluntarily walk out and drop their doctor. You can call this a sense of self-preservation and preserving ones independence. No one wants to lose their freedom and I don't blame them. However, there comes a time when a little extra help is necessary. Be prepared for a very big fight, this is common. Anytime you go to take away someone's freedom, you're in for a fight because they'll become combative, and I don't blame them because I wouldn't want to lose my freedom either. If there's some way to bring in properly trained professionals to keep them in their home and make it work, this will keep them in their homes as long as possible and out of a nursing home. As for the stealing though, make sure whether or not someone really is stealing because sometimes yes, people will steal even from dementia patients. If they have assets, someone who becomes POA could end up taking advantage and turning things in their favor to make sure they get whatever it is there after such as a house, car, money including making themselves beneficiary to insurance policies and the list goes on. People take advantage of our elders if we don't have someone trustworthy protecting those items for the person who's no longer able to fend for themselves and is now sitting duck for predators. This is common and can happen to any elder if we're not careful because it will rob rightful heirs from their inheritance. A family crisis is already hard enough on the whole family without someone coming along and taking advantage of one of their own especially if that someone is not a family member not entitled to anything but they tend to take advantage of others who are vulnerable. Be very careful that this is not the case because the dementia patient could very easily turn out to be right

I whole heartedly agree with other commenters. It is very difficult to get a doctor to prepare a competency document and sign. They may diagnose with dementia and recommend assistance for daily living; but getting a competency sign off is very difficult and unlikely from primary care physicians unless there is a crisis for example a fall, stroke, etc.

You can however start documentation by keeping a diary and documenting dates, times of accusations, paranoia, hallucinations, memory failures, lack of hygiene, pAying bills, odd behaviors, etc. send copy to dr about a wk prior to appt. make sure when u make appt that you request physical PLUS mental health work up. Likely dr will refer you for follow up with neurologist and you can go from there.

As long as your sib doesn't believe u are stealing from parent you should be ok. Keep recording the accusations from dad as a matter of record.

I have same problem. 94 yr old mom with dementia. She first accused distanced brother of stealing and then for last 6 yrs it's been me as I'm around her more and make contact with her. The closest child usually bears the brunt. This is common behavior for most elders.

Before you go incompetency route, make sure you've done your homework and have a plan for dad as to where he will live, financial considerations, in home help possibly, etc. and for heavens sake think long term and costs. You and sib need to be on same page or there will be heartache.

You can facilitate the process by bringing in written documentation of all difficulties you've noticed your father having. Check online, there is a list of 8 behaviors that family caregivers can make note of, and these often correspond with dementia.

Also bring in written documentation of what others have observed. Your sibling may be reluctant to do this, but if there are others who often observe your dad's behavior, such as neighbors or other family members, bring in that information.

As a doctor I can tell you that it's a huge help when a family comes in with a record of what behaviors and problems they've observed, and the more observers, the better.

Delusions of theft and other paranoid behaviors are extremely common in older adults, and are often triggered by cognitive impairment. That said, I try to not dismiss such worries out of hand, because occasionally it does seem that someone is stealing from the older person.

Good luck.

Where is your father living currently? You didn't say. In the various facilities it's common for one's belongings to go missing. A friend of mine was in such a place and sadly, many of his cherished belongings were stolen. CD's especially! I don't know why these are stolen so frequently, perhaps because they can be easily stashed and resold. Same thing happened to my grandmother. I heard the jewelry stolen was not highly valuable but it was hers and it meant something to her.

I absolutely cannot imagine how unnerving it would be to have that worry hanging over you. You start to hide stuff and get fearful, not knowing which of your belongings will "disappear" next. The stress can turn to paranoia and unfounded accusations. It's a reflection of feeling powerless. If you are actively trying to get him declared incompetent, you bet he will be extremely fearful and defensive. Who wouldn't? Imagine him feeling like his family, that he raised, is against him and wants to take control of his life, even have him put away.....

As for the will, that's another story altogether. Since right now he is able to converse (enough to get accusatory) how about confronting him directly? Bring in extra support to get a perspective on it. It's better to iron out differences now than to end up in legal battles after he dies.

Your father's physician may examine him or send him to a neuropsychologist for a more thorough workup. In its early stages, the progress of dementia can sometimes be delayed. In addition, a diagnosis valid at the time your father signed his Will is important to show that he lacked legal capacity to make it.

Are you in the UK, David?