She has medical issues herself with uncontrollable diarrhea and now he is becoming incontinent. She has always had him waiting on her, hand and foot. Doing all of the chores or projects she comes up with. But now he does not even know what a screwdriver is much less how to use it. She keeps asking him to do this or that and then gets mad and hollers at him when he gets or does it wrong. I ask her why she does that when she knows he has dementia and her response is that she keeps hoping he will rise to the occasion. WHAT! REALLY! I think she knows that there is no one else on this planet that is going to kiss her ass like he has always done so she doesn't want him to go.
My message to you is 1. hugs and sympathy, sister!!, and 2. it's time to start getting the ducks in a row (healthcare and financial POA papers, will, etc.) and the options lined up for care for both of them. The emotions surrounding all this are very, very hard. Give yourself time to adjust and get all the support you can from friends and family.
And you're not exactly what one would call classy with your "a** " comments either. In fact your post is quite hateful in the tone it is written, maybe it was done in a moment of anger. I realize not everyone had the kind of parents I had, but there is a lot of venom in your thread.
This is really just a question of sooner or later. It is so much better to be sooner.
Focus really tightly on his best interests, first. Hypothetically, you find him a really good, really nice dementia care unit and by hook or by crook get him in there. The question is, would he be happier? You and I might be; but he'd have been removed from the wife he's served all these years: it's sad to think of, but he might find the loss of that atmosphere that you find so stressful more painful than anything else.
That doesn't mean things can continue as they are. He is no longer able to support your mother, and clearly she is going to need support if she doesn't already. So either care has to come in to the home, which at least will mean a new layer of protection for your father, or they both move to a supported living environment.
If she's not budging after you've pushed for a while, and it's getting to the point where her treatment of him is unmistakably abusive, you're going to have to confront her, stony-faced, on that point and tell her you're getting them assessed whether she likes it or not. Then act on it. So do your homework, get your social workers, OTs and the like lined up; and then best of luck to you. I agree that something is going to have to change; it's a question of making as sure as you possibly can that the change feels like an improvement - for both of them, ideally, not just your more deserving Dad!
My Mom still thinks my Dad can do everything he use to back when he was in his 50's... now at 93 Dad has issues with seeing and with balance, so fixing anything is quite a challenge, and he makes whatever he is trying to fix even worse... [sigh]. Plus Mom will send Dad outside to help me unload my vehicle of groceries... good heavens how is he suppose to carry a heavy bag of groceries and also try to use his cane at the same time. It's all denial on both sides.