Dorothy, I hope talking/venting/ranting about the issue of your daughter in our weekly Zoom chat was helpful for you.

Based on your posts here on BCTS I was under the impression that your daughter was way younger than she is. My impression had been that she was maybe just starting junior high. But given the fact that she is a non-emancipated adult, and that you are NOT her guardian, I want to point out a few issues that occurred to me on further reflection after our chat.

First of all: You need to take care of yourself first! Jesus said: "Love your neighbor as yourself." If you do not love yourself, you can not love your fellow human beings. So in order to take care of others, you need to take care of yourself.

Set clear boundaries and limits regarding the use [and abuse] of your resources. That includes time and your mental and spiritual well-being. If spiritual fellowship is important/essential for your own well-being, then you should only miss those fellowship meetings under the most dire and sporadic exceptions (e.g. once every three to six months). Do NOT miss your own spiritual fellowship just to take somebody else "church shopping" all over the damn city or province. If they do NOT want to accompany you to your congregation, then let them make their own arrangements of transportation. Just as you do not force other to come to your congregation, do not let other force you to go to their congregation!

How is the fact that you are the primary care-giver for your mother affect your ability to take care of yourself? Are you the sole carer, with sole responsibility and authority for the affairs of your mother? Or do you share that authority and responsibility with your siblings? Is the sharing happening in a spirit of mutual support, or is it more in antagonism and strife? In general, the in-laws SHOULD keep out of the arrangements made by and among the siblings! If the in-laws cause [negative] stress in the arrangement (because the try to gain some personal benefit), then you need to set clear limits and boundaries to your siblings and request [or demand] that they call their spouses back! Also spell out the consequences for not respecting the boundaries and limits you set.
Worst case scenario, negotiate and have a legally binding contract drawn up by an attorney/notary that spells out expectations, responsibilities, boundaries, authority and limits, as well as the consequences of non-compliance or interference. For example an interfering sibling (and/or their spouse) might be required to become the primary care-giver. A prospect that might help to keep busy-bodies in line.

What is the outlook on your mothers mid- and long-term condition? What is the expected consumption of your resources due to the care required by your mother? Will you be able to cope long-term as the sole/primary care-giver? Or will you eventually need financial and/or manpower support? Based on the hints you dropped in your posts here on BCTS, I believe that the care for your mother might become to big of a strain for your own mental health. So it is very important that you set very clear limits, boundaries and expectations for your siblings AND their spouses!

Given that your daughter is legally an adult [albeit non-emancipated], your primary responsibility towards her as a parent is over and done with. Though she is still your daughter, the responsibility dynamic has shifted. You can only ask, request, recommend, beg (in other words, NAG), but you can not force or impose things. Though you can make some requirements for the exchange of your resources.

You should probably get some competent legal counsel before you intervene in non-emergency health-care of your daughter, as that could be seen as interference and/or involuntary treatment by her guardian. Which could probably lead to criminal charges. And where would THAT leave both your daughter and your mother?
Is it possible to have joint guardianship in Canada? Do you have the [mental] resources to contest the guardianship of your daughter? Would you be able to cope with the additional responsibility of care-giver for your daughter, on top of your mother?
Eventually you will need to make separate arrangements for the care of your daughter, because statistically parents predecease their children.

Very clear and strict boundaries and limits are all part of tough love. And tough love is hard on both the lover and the lovee.

Wishing you all the best.