What can I do when my husband screams and yells at my MIL all the time but she has dementia?

CP, I've been reading over your past posts, you are located in Ontario Canada, right? You do realize that basic long term care is less than $2K a month here, so sibling's worry about keeping an inheritance shouldn't even be an issue.
What happened when you contacted CCAC (now your local LHIN)? If you are stymied by lack of POA then work at that angle, but the only time I have been asked to show my paperwork was when mom was being admitted for care.
Since we do not have an equivalent to APS in Ontario you may want to explore this route:
Office of the Public Guardian and Trustee (www.attorneygeneral.jus.gov.on.ca/english/service_standards/service_standards_opgt.php)
Anyone may contact the Investigations Unit of the OPGT to report that they believe that a mentally incapable adult is at serious risk of harm. The telephone number is 416-327-6348 or toll-free at 1-800-366-0335.
Once a report is received, the Investigations Unit will respond by trying to contact the person who made the report to gather more information. This first step will help to determine whether the OPGT will initiate an investigation for the purpose of deciding whether the Office should apply to court for an order of guardianship

Woooohh.

I know he doesn't intend it this way. I know you must both be at your wits' end over this. But what your husband is doing is uncomfortably close to being not just borderline abusive, but actually abusive.

I yelled at my mother too. She drove me bananas, and as she put it I have my father's temper. But that was before her dementia became apparent, and dementia changes everything.

You have done the right thing, which is to take the first step. What you need now is advice from somebody who's on the spot and in a position to help you. Your GP, a social worker, a local day care centre. Just anybody professional with the right network and the right skills to intervene constructively.

Impress on your husband that NOBODY who has been there is going to blame him, or fail to understand why he doesn't want to talk and why this is all so hard to face. You could do worse than refer him here, you know - there are several male caregivers on the forum, he won't be the only bloke.

Its time to find a place for her. Ten years is a long time to care for someone. Your husband is tired. Who has POA? He needs to get his siblings together and tell them he is no longer being the caretaker. Someone needs to take over or find a place for Mom.

This was a large factor in why I ultimately why I placed my mom in a NH, and I never reached the extremes your husband apparently has. It is unfair to her to have to live with abuse, and just as important, it is unfair to him to have to live with such extreme stress. Ask him, is this the way he wants to remember his final years with his mother? Does he want this behaviour etched on his soul for the rest of his life? He is feeling trapped because he will not consider other options, but they are available. If contacting APS is the only way to force change then I feel you are not only justified but obligated to do so, in my mind it is just the same as contacting children's services if there is child abuse, both are equally vulnerable.

Most answers here seem to think the husband can be reasoned with to change. That's not going to happen. Adult Protective Services must be called right away. This is elder abuse! It's a lose-lose having her there and it's a powder keg waiting to be lit. Get her out of there and now!

It is time for hubby to learn everything he can about Dementia. Hubby is trying to "fix" the situation, and he's frustrated that he can't fix this problem, so he yells.

Scroll down to the bottom of this page to the blue section... click on ALZHEIMER'S CARE... now see all the articles, read as much as you can, and let hubby read them, too. Maybe then he will realize what he is doing is only confusing his mother even more.

Another good article, https://www.agingcare.com/articles/not-everyone-cut-out-to-be-a-caregiver-162192.htm which was a life saver for me because I thought something was wrong with me.

If none of this works, does Mother-in-law have enough in savings to move into Assisted living/Memory Care? The cost is around $6k per month, depending on where you live, it could be less expensive. That way your hubby can once again be the "son" and not a "caregiver", and it would make life much calmer for you all.

Yelling and not handling his mom's needs has to stop. If he isn't interested in getting her proper care, I might let him know that he or the family will do it, or someone else will. I'd seek a legal opinion from an Elder Law attorney. Anyone in the family, including you, can file a legal action in the court to have a Guardian appointed. The court can appoint a proper person, which would not be your husband, to make decisions regarding her care, living arrangements, healthcare, finances. If no one in the family is suitable, they can appoint some other individual or even the county adult protective services. It might cause a riff with your husband, but, I can't see me allowing that mess to go on. It's just not right, imo.

Can you call her family doctor? Or his, even? Don't forget, when this is having such a terrible impact on you too you really don't need anyone's permission or agreement to seek help.

I am in the exact situation. We "hired" my brother-in-law to be caregiver along with myself. I moved back from Florida to take care of them. Unfortunately, since I've been back in NJ, I have had 2 cancer lumpectomies, radiation, spinal stenosis surgery, knee replacement and pneumonia--ALL IN ONE YEAR!! I never even went to the doctor's before except for routine checkups. So I needed help and am not physically able to do what mom and dad want to do. Oh, yes...my mom has moderate dementia, my dad has severe. It has now been 2 1/2 years and I am with them 24/7. Like you said, your husband yells and it sounds like my BIL. He absolutely refuses to believe my mom can be so forgetful, embarrassing, loses everything, can't hear well, teeth fell out, had to be replaced. She lost 3 sets of teeth and "cleaned" her hearing aides by soaking them in water with her teeth. That's just part of it. My BIL started out amazing. Don't know how we did it without him. But now, all he does is sulk and tell off my mom. He won't let her go outside on these beautiful days to work in her garden or water flowers (she does that 2-3 times a day. Drives my BIL crazy. He feeds us, but won't talk to anyone. It is very depressing for all as we were always a family with a humorous disposition. So YES, I feel it IS abuse, and I tried hiring HHA. My SISTER who is never even here, spoke to my mom and talked her out of it because SHE needs the income. I've been here 2 1-2 years as I said, and NEVER gotten a cent. This is supposed to be HIS job, but I'm not sure I want to leave them alone. So I do more than he does. But I feel he is abusive , no one will listen to me, so I am now looking for a place to live. I am on disability for depression and anxiety, yet I got thru this without ever losing my temper. I can't do it any more, so I'm passing it along to whoever decides to take over, My heart just aches!

Thank you so much for this information. I will definitely bring this home to my husband. I admit that his behaviour is turning into downright abusive behaviour. For the sake of his mother's health AND his health it has to stop. 

I wasn't involved in the last time LHIN was involved. A case worker determined that she's eligible for home care. Nothing was ever done.  No paperwork was ever filed.

I'll be the first to admit - she was never one of my favorite people, but I wouldn't even yell at a rabid dog the way he yelled at her yesterday.