Will it be easy for my husband to take care of his very selfish mom in our home?

You said no.
Your no was not respected.
I would take that seriously.
I would schedule marriage counselling asap. It can really help to have a third party to help each of you to have your say & be heard.

In the meantime, ask him to move out. He can look after his Mother if he wishes in his own space.

Think about your own question that you're asking here.

'Will it be easy for my husband to take care of his very selfish mom in our home?'

Here's the answer to your question:

What do you think?
It will be more than hard. It will be absolutely miserable. If you think that you won't be taking care of her, think again.
My friend, It will not matter one bit if you say no or refuse. She's in your house. When she craps her diaper, you will change her. When she's hungry, you will feed her. When she's dirty, you will bathe her. When she wants to behave abusively (verbally or physically), you will take it, and when she calls, you will come.
I've been an in-home caregiver for almost 25 years. I've worked for couples whose marriages ended because an elderly parent needing care was moved in against the wishes of one spouse. Many wives flatly and adamantly refused to do any care for a MIL or FIL.
Yet, the first diaper or pull-up that gets crapped in is changed by the wife. The first bath or shower - The Wife.
Mark me, this is what will happen. Men are allowed to use the excuse of 'just not being able to do it' or 'she's more comfortable with a woman helping her' to get out of the gross care and it's accepted.
You will be lucky if the caregiving gets pushed on you gradually and politely like BlueEyedGirl94 says. Or you will just be the Designated Caregiver from day one. Now I want you to think about what your house will become. The needy, incontinent, gross, elderly invalid who hates you to begin with, will take over the main living area of the house. They will commandeer the living room. Take it over like Emminent Domain. This is almost always happens. Their junk, meds, assorted crap and kleenex will be everywhere. There's always kleenex in the furniture and all over. Crumbs and food too because they'll be spending all day out there and taking meals. Get used to a place that smells like piss and sh*t all the time too.
This sounds unreal, but I have known many DIL's whose MIL's hated them and yet were moved in. The MIL's would crap themselves the minute their DIL's walked in the door. They'd hold it for them. I wouldn't even be out of the driveway yet. They'd wait for their DIL's. Even the ones who still used a toilet and were only in pull-ups for 'accidents'. There would be daily accidents for those poor DIL's.
As a spouse you have legal rights and do not have to let her move in your house. Speak plainly to your husband and tell him it's either her or you. If he moves her in against your wishes, meet with a divorce lawyer. Good luck. I wish you all the best.

Caregiving takes over the whole house no matter how large or well-appointed it is. Who cooks for her, shops for the food and many incidentals, like chux pads, she needs? If it’s you, you’re caregiving. Who watches her when he is out? Who carts the dirty diapers to the trash? If it’s you, you’re a caregiver. Who cleans the carpet when she vomits or pees on it? Who washes all the extra clothing and linen a sick person uses? Who converts the toilet to a raised seat? Who struggles to get the wheelchair through a too-small door in the bathroom? Where do you put your home items like the chair that is displaced because she needs a Hoyer lift? Will you feel comfortable having friends over when your house smells of urine and poop? Where will husband be most of the time? What happens to your marriage? These are realities. Caregiving is hell, not a fun family experience. If you go along with this, be prepared for all of the above and more.

No, it will not be easy for him. I'm guessing that he assumes you'll jump in and help despite what you've told him.

Be prepared when that time comes (and you know it will) by making a list of in-home care providers (aides to bathe her, somebody to cook for her or meals on wheels if he won't cook for her, etc.)that he can call to care for her and hand it to him when he asks. If you can go to another part of the house where she isn't, all the better. She doesn't sound like a nice person to be around.

He made this decision alone, he can handle it alone.

Will it be easy for him...NO.
Should you help make it easy for him...NO.
You continue doing what you have always done, do no more than that.
You do not indicate your age, how long you have been married or if MIL has dementia or other medical conditions or if you have kids at home or not.
I do think you might want to talk to someone about this. Personally I would be real close to packing my bags. (after talking to a lawyer) I think this shows a blatant disrespect for you.

I wouldn’t put up with any of this starting now. I would see a family law attorney tomorrow about getting a legal separation, particularly if he’s the only one bringing in income.

Im assuming you two have been together long enough for long term marriage statutes to apply in a community state. In which case he can finance your living elsewhere while you work out what’s to be done with the house and with provisions that your half aren’t to be drained for mommy.

Go on a cruise alone for a month. When you return if he still insists on her being there file for divorce.

Whar he's pulled has got to be among the most disrespecful things a spouse could do.

First thing if you suspect your marriage is heading for the rocks, is to separate any joint accounts. It has become a standard practice to drain joint accounts to leave the other person penniless. An equal split is at least fair. Just get there first.

Second thing is to work out and write down what you will do and WHAT YOU WON”T DO. You won’t change her bedding, her diapers, or her clothes. If she’s dirty, she stays that way until DH cleans her up. You won’t launder her dirties either. You will cook her meals (perhaps). If so, she gets a serve of whatever is being cooked, not a la carte on demand or at times of her choosing. You will not serve her meals at the table with you and DH. She eats in her room. You don’t pick up the dirty plates and wash them up – she eats off paper plates if necessary. She does not share your living room after dinner - DH provides her with a small table, comfortable chair and TV in her bedroom, and that’s where she sits.

Spell it out to DH. Have the argument BEFORE she moves in. Argue about the big picture, not sniping about individual actions. Write it down and KEEP the copies.

If DH doesn’t provide the care he is offering, and MIL’s living conditions are unacceptable, call APS yourself. Make it clear that you spelled this all out in advance. That’s why you need the copies.

Fight for your life! Because otherwise you won’t be living the life you chose.

Decided to bring her to your house from where? Where was she living at the time? What prompted him to take her away from there?

What do you mean by: most rest homes have refused her because of her terrible bad behavior toward the staff? If they refused her, as in refused to admit her, she can't have had an opportunity to upset the staff. If more than one facility has asked her to leave because of abusive behaviors that's a different matter, but such behavior would have to be quite extreme for a step like this to go further than mutual agreement.

Anyway. What actually matters more is:

Do you like your husband and want to keep him? If so, you need to help him find more practical options for his mother and support him in withstanding emotional blackmail and the like. Don't refuse to contribute to MIL's care because all that will happen is that he will feel abandoned and trapped and stressed and he will become even more enmeshed with his horrible mother. If you lead or at least share the care, at least you'll have some control over what's going on (including, for example, sourcing outside help and deciding next steps).

If on the other hand your husband's ignoring of your feelings and wellbeing is all part of a pattern and you've already had enough of this marriage to last you... Is it time to walk?

Everyone brings up good points.

Would it be possible for you to take a little two week vacation....maybe visit the kids? Leave him alone to his decision so he gets the full impact. Whenever my grandmother visited my father would use my mother and I as a 'buffer'. Then one time he had to spend the weekend alone with her at her house. The man came back looking like he did a tour of duty in Vietnam. "He had no idea she was so bad". Sure he didn't because he always left us to deal with her.

I agree with most everyone here. Don't start trouble but stay out of their way. The most I would be doing is making extra for dinner, but he is the one who feeds her and cleans up after her. He is hoping you will jump in. If it ever gets too intense, just pick up and go to a movie. Is he really ok with bathing her? Toileting her?