Am I wrong to not want to care for my mother for the next 25+ years of my life?

I want to add to all the good suggestions here that your mom should be placed in some kind of supported housing environment right now, e.g., group home, while your grandmother is still alive and can be a support and comfort to her in the transition. Too many people have kept an intellectually challenged child at home until they themselves become too infirm to manage their care. It's terribly hard for both the elderly parent and for the adult child who is suddenly thrust into an "alien" environment. Has your mom every gotten any services for adults with intellectual disabilities? If she hasn't, it's not too late to start getting this set up now. You should not be "IT" in this situation, for absolute sure

I hate to be the bearer of bad news, BUT.... your mother only being 54 years old, could live well into her 90's which would make you caring for her another 50+ years, not 25+ years.
It sounds like perhaps your grandmother has somewhat enabled your mother not allowing her to live on her own or in a group home of some sort to give her more independence.
You don't really explain what kind of "intellectual disability" your mother has, but allow me to say this as plainly as I can.
YOU ARE NOT RESPONSIBLE FOR YOUR MOTHER!!! Period. End of sentence.
When your grandmother decides that she can no longer care for/enable your mother than it will be up to her to make arrangements to have her placed in the appropriate facility where you will just get to be her daughter and advocate(if you choose to).
And you can live and enjoy your life any way you see fit.
And hopefully it will be like Alva always says and I have to agree, at least 1000 miles away from your mother.
It's time to find your voice and let your mother and grandmother that you WILL NOT be taking care of your mother....EVER!

No, you are not wrong at all to want a life of your own. Why did your mother have children if she has an "intellectual disability" that would require caregiving for life???? Can she not go live in a group home type environment where she'd have support? Perhaps you can speak to a social worker about such a possibility for mom. You'd not be "abandoning" her, just living your own life and allowing her some autonomy of her own.

Best of luck to you.

You are not obligated to take her in. No one else wants to do it, so you've been made the scapegoat. This is not okay. And you are not wrong for not wanting to give up the next 25 or more years of your life. Just because they expected you to, does not mean you must oblige. Everyone else will be happy while you are drowning. How is that fair?

People tend to think that if someone is not married or has no kids, that they have all the time/money in the world, and can take on anything family dumps upon them. Or you have nothing else going on in your life that takes up time.

Mom would do better in a group home for intellectually disabled people. I have a relative who lives in a group home and overall is happy there. If she is able to work, there's programs for that too.

You have to stand your ground NOW. Others will try to guilt you with "but it's your mother, you have to do for her!" and "you're being selfish". You're being just as selfish as the aunt who won't take her in because her husband wouldn't like it....and yet she likely doesn't think she's being selfish at all. Maybe you’d like to get married someday too? Why is it okay for the aunt to insist she can’t sacrifice her marriage, but it’s fine for you to sacrifice your future marriage?

Start finding arrangements for mom. Do not move her in. Not even as a last resort.

There is no way you should be responsible for your mother’s care, it’s simply wrong for anyone to ask or expect this of you. Your mother does require ongoing care and supervision, and the place for that in an adult with intellectual disabilities is a group home or supported living setting. It’s not an institution, thankfully those are now rare, or a nursing home. Your mother can live an active life in a supported setting, with activities, possibly a sheltered employment job, and a social life with others with similar needs. You can visit and be her advocate. Please let all family members know this is the next step, one you’ll assist in finding. Find a chapter of ARC nearest to you, or call the local Council on Aging for a referral to the appropriate services. Do this sooner than later as there may be a waiting list. Don’t bend on this, it’s important for both you and mom have a future that meets each of your needs

The job description for Sainthood isn't very good.
The outcome in life for Saints has a history of not working out well for them.
You have a right to your own life.

If your mother is incapable of living on her own then you need to contact Social Workers through her medical team and learn how she can apply for Medicaid (assuming she has, with her disability, nothing other than SSDI to support her) so that she can be placed in a group care home.
There she will have QUALIFIED and trained personnel to care for her. You will be a loving son or daughter visiting.

I often say that the best place for family is about 1,000 miles away. In your case I truly mean this. You should move out of the area in which your family is living and make a new life somewhere. You will otherwise end up for your lifetime an unpaid slave to family.
Others may have "expectations" for you. That isn't important.
You have expectations, but you also have CHOICE.

That is my personal opinion. You are now a grownup. You will make decisions for your own life now. That is your freedom to do, and your right. Whatever choices you make for your own life now I wish you the very best. Remember, you did not cause this and you cannot fix this. You can, however, sacrifice your own life to it if you CHOOSE to. Remember, it is a choice. There is no answer. There is nothing here that will change anything nor make anyone happy. It is simply a tragedy, and I am so very sorry for it.

Explore options. No, you should not care for Mom for the next 25yrs. Contact your county Disabilities Dept and see what is out there for someone like Mom. There may be a nice group home that she can be placed in now.

There was a family when I was growing up who had a Downs syndrome daughter my age. When she was approaching 60, Dementia set in and she started becoming a problem. Her mother moved both her and the daughter into an AL. It was promised if Mom died before daughter, the AL would continue to care for the daughter. The daughter passed in her 60s, before Mom.

You should not be expected to care for Mom. She probably was more a sister than a mother and you should not be expected to care for her.

Is your grandmother her legal guardian? Is anyone? Whoever is her legal guardian should start talking to a social worker for her county. If she doesn't have any legal representative, then the social worker would need to know this. A judge can assign her a legal guardian who will meet all her needs going forward. Start the process now as it may take a while.

Just because your aunt and grandmother insist you do this, it does not mean you have to. That is the beauty of being an adult....you get to decide what you do and don't do, not the family elders. Remind them of that fact.

You are not wrong in thinking this

any "agreements" you may have been talked into as a teen do not have to stick now that you are an adult. You can make your own decision as an adult about how much you will do