Any ideas on how to set boundaries with siblings that won't help care for my 85-year-old mother, who is very often abusive?

Does she have dementia? If so, let the comments roll off your back.

If you've decided to take on her care, then you are going to have to be less sensitive for your own sanity. You know what she's saying isn't true, so why get upset about it? I'd have laughed at the one about your brother and said "That's a good one, Mom!"

That would take the wind out of her sails. You can't try to extract empathy from someone who doesn't possess any, so turn the tables on her and take what she says as a joke.

It sounds like your siblings have set their boundaries. They have decided they will not provide care to an abusive old woman.

My heart goes out to you & anyone raised by a narcissistic mother. I share your tragic & difficult upbringing & understand the wounds are deep as is the guilt. No one will fully comprehend your situation unless they have personally gone through this experience.
I have found with my mother who is turning 85 (I am turning 64) building a network of resources, finances permitting, takes off a lot of pressure. I have hired a housekeeper, groceries & meds are delivered etc.. I have found a woman at her church who enjoys driving her to appointments & on errands. My own journey of retaining my dignity & sanity was to relocate 3 hours away. I have traveled to care for my mother when she is very ill, but the moment she starts recovering, the obnoxious demands & unpleasantness returns.
Narcissists must be in control at all times because internally, they are out of control. Ugliness is their method for putting & keeping you down so they can feel elevated & in control. When this happens, I go home. My siblings, 2 brothers, one out of state & one 6 hours away, do not participate in her care & I have accepted there is nothing I can do to change that.
I have told my mother the next time she is very ill, she will need to call an in home care agency. I have reached an age where I need to protect myself & my health from the stress, abuse & physical exhaustion that comes from caring for an aging narcissist.
I too am a Christian & believe honoring our father & mother has no time limit. The Bible is very clear in defining bondage. It is our right & responsibility to recognize our God given free will to make the best possible choices & decisions.
Love & Prayers

"I want to be obediant & honor my mother" 

Obediant: submissive to another's authority.

Why do you want this?

Is it always wrong to question authority? Wrong to fight unjust laws? Wrong to fight for our human rights?

Ever wrong to question the authority of a parent? When a child? When adult? When the parent is mentally ill?

Another interesting word: Honor: high respect, great esteem.

Thinking of someone with respect, with esteem takes place in your mind.

I respect many people yet I do not offer them servitude.

What does honor mean to you?

"She is almost 85 lives independently". (From profile)

Have a think about this statement.

IS Mother *independent*?

I found two examples to define independence;

1. free from outside control; not subject to another's authority.
2. not depending on another for livelihood or subsistence.

Mother may indeed pass 1. but 2? She seems quite DEpendant on the OP.

Would "She is almost 85, lives alone" be a better fit? Even "She lives alone, dependently".

You may wonder if I am some sort of nit-picking word obsessive..

Sometimes a good look at the 'facts' is needed. To look into the statements we believe & ask if they are even true.

Then we can start to see things a little differently.

The is the first of many.

My heart aches for you - and that is straight up truth - because I see so much of my SIL and my husband in your post. They are slowly coming out of the FOG (Fear, Obligation, Guilt) of obeying and honoring an abusive, narcissistic father who believes they owe him fealty and servitude after years of horrible abuse. I recognize that it is easier said than done to break free from the years of conditioning from verbal abuse, especially since your mother had all the time in the world to get into your head and ensure that you feel like you owe her - pair that with the strong religious ties ( I get that too). But please hear what I'm about to say - it will hopefully save you some pain and help you move forward.

You cannot earn what she cannot give. It does not matter what you do or how hard you work. If she doesn't have it to give to you, nothing you do will change that. If she has always been this abusive person, she spent years conditioning you to believe you were supposed to take care of her (and it sounds like she did). What about YOUR life? You have a right to a life. You are your own person. You should be able to live a life free of abuse. YOU DO NOT DESERVE TO BE ABUSED. Abusers thrive on and COUNT ON the fact that you will stay and continue to take their abuse. And in the case of parents who abuse - they have taught you that you deserve it. You don't.

Just because she is your mother, you do not owe her the right to abuse you. She is provoking you to wrath. It's time to consider yourself. And other options.

Here is what I will tell you briefly in our case. The more my DH and SIL stand up to FIL - the smaller he becomes. The less power he has. He blusters, he yells, he gets angry, he is MEAN. But guess what....that's all. He has words....but once they realized that he couldn't hurt them...he had no actual power....the words did nothing anymore. There is a silly movie where Scrappy Doo is this big evil monster because he got some bad magic power, and the team takes the magic away and he's fighting them and doesn't realize he's tiny again and still fighting and snarling and that's how I see my FIL now...this snarling little mess who keeps fighting but doesn't realize he doesn't have any power anymore. It's kind of sad but also empowering to DH and SIL because they realize THEY have the power now.

Take your power. Realize that YOU really do have the power here. What happens if you stop taking care of her? Is there anyone else? Because that IS power. If you aren't doing for her - she has to pay someone or go somewhere - that makes you important - REALIZE that. SHE NEEDS YOU. Start there. You aren't expendable. Seriously - You need to realize that. If you aren't willing to seek other options right now. You need to at least recognize that she needs you more than you need her - that shifts the power. Don't wilt when she says mean things. Tell her. Tell her that if she doesn't need you, then you are going to take some time off. And then do it. Watch what happens. For the record, my FIL PANICS when my SIL leaves the house, just an FYI. So you need to recognize your worth here. That is your currency to start setting your boundaries. YOU need to start with you.

She is abusive because you allow it and keep coming back for more.

Do you have a pastor you could speak to about this?

I doubt any mainstream Christian clergy would tell you to "obey" your abusive mother.

It seems to me that your husband is your primary responsibility.

You say you are a Christian and want to be obedient and honor your mom.

You are not a child, when you got married, you left your mother and father and became one with your husband. Nothing in Scripture tells us to be obedient as married adults. Honor is respect, not accepting abuse.

Scripture also tells parents not to provoke their children to anger.

If you are going to use Scripture as the why for accepting her abuse, please research it and use actual Scripture.

I hope you can follow through in putting more emphasis on YOUR needs. Is there just one sibling -- Golden Boy? How incredibly hurtful when your mother says things like he is the only one she can count on! And she hadn't seen him in almost a year?

My mother said similar things to me. She fawned over Sonny No Show, said HIS time was valuable, yet mine was worthless. Yes, he lived out of state, but wouldn't bother to travel to see our mother. Interestingly, this was not her favorite child while he was growing up (I have 3 brothers). And all I suggested was that HE research for one of her wild goose chases instead of me.

Oh, and she told me I was too sensitive, also.

Free, guilt is such a useless emotion.

Your mother has made her bed with a lifetime of manipulation and abuse. It's not your job to care for her. Children are not a retirement plan.

"I had spent several hours shopping for her and she asked me if I had heard from my brother. I responded that no I had not talked to him recently and she had not seen him in almost a year, but she described him as the only child she had that she could depend on."

Sometimes you need to fight back. You could have firmly said "you can depend on him? Good, because that means I can take a break. So next time you need groceries, call him"

When she starts the verbal abuse you walk away. The next time she does it, tell her that you will no longer take the verbal abuse. She appreciates nothing you do for her and you don't have to do it. So, maybe next time call golden brother she can depend on. Then walk out of the apt. Do not take any of her calls for a day or two. When you do, if she gets abusive, tell her your are hanging up and until she talks to you like the adult you are, you will keep hanging up on her. People treat you the way you allow them.

 "No one can make you feel inferior without your consent." Eleanor Roosevelt

You say your siblings don't wish to help in care of your abusive mother?
That's fine. They don't have to do anything. Neither do you. Each person chooses how he or she will help or not help.
Other than that I am uncertain what boundaries you are wanting to set, and who you want to make them clear to.

Your profile says that mom is wheelchair bound for many years.

AND that she believes that no matter how abusive she is, you will never leave her.

What medical and or psychiatric conditions does she have? Has she been diagnosed with dementia?

Where does she reside?

To what extent do you "help" her? Do you live with her or she with you?

Have her care needs been professionally assessed through a "needs assessment"?

What are her financial resources?

You ask about boundary setting; this can be saying "mother, you may not speak to me like that; I am leaving now".

It can be saying "no, I can't possibly do that."

It can be arranging for her to have professional care, either in a facility on her home, paid for by her.

Unless you are able to define “I never will leave her”, are you really focusing on “setting boundaries”?

Do you hold her POA?

HOW is she “abusive”?

If your siblings are willing to limit contact with your mother, do you have a reason for saddling yourself with her care? Is there some reason why you feel more responsible for her than her other children?

Why are you the one taking care of her? Just because the other siblings refuse?