I’m sure this is very, very frustrating. Your father is fortunate to have family who are concerned. But that concern doesn’t have to translate into giving into his demands.
I’m guessing your father is in his late 60s. Although I have successfully worked with many seniors as they adjusted to major changes like the loss of their spouse, they, unlike your father, wanted to make changes. Your father doesn’t. He has had an entitled life for many years and sees no reason to give it up. It doesn’t matter how “right” you are. You aren’t going to reason with him into behaving differently. He’ll have to experience the lack of help in order to finally get it that his comfort depends on him.
All you can do is state the reality that it’s up to him to take care of himself, give him choices, and then leave it up to him. There’s no need to argue. There is no need to feel guilty. Stay friendly. If he starts complaining, calmly remind him that he is capable of being independent and change the subject to an update on kids’ activities and family news.
The one caveat is if the “mental issues” you referenced are legitimately getting in his way. In that case, you may need to convene a meeting with a counselor for yourself and your aunt and uncle to talk about how best to care for him. If he is, or becomes, incapable of caring for himself, then the family needs to look into what resources are available to provide support for all of you as you try to provide care.
I wish you well.