Oh boy, I feel for you. And I can relate. We are in the thick of these issues. My husband's parents are completely irresponsible and clueless about money (not saying this is your M., just our situation). We know they are completely in debt. A few years ago they called on a Friday and basically said they hadn't paid their mortgage in a few months, and lawyers were calling and if they didn't have x amount of dollars paid by Monday they would be out of their house. We were so mad, of course we can't let them live on the streets, and that was no time frame to come up with a plan of action, so we took money from savings to pay it. My parents are actually responsible, but are elderly and not in great health, and they lost a TON of money they had invested due to the market crash how ever many years ago. So they live very close to broke and don't make the greatest decisions because of their age and health.
So we have been stressed about what to do. Of course we love them all and know we need to help- but how and how much is always a concern. What we always tell each other is that if we spend more than we should on them, we will eventually be the same burden to our daughter and we refuse to do that.
So long story. But I'm telling you this because it led to our eventual decision. We spent so much time saying "didn't they see these money issues coming?" and then realized we were doing the same thing. Don't we realize that if they have no forthcoming income and can't seem to make their bills, they will eventually need our help. So we started putting money aside in our savings account strictly for the purpose of helping our parents. We do not tell them about this money because then they will want it right now!
At different times,. we have told our parents we aren't able to just give them money for this or that. And we've said that if they are in need of something, they should mention it and we can apply money toward it instead of giving Christmas or birthday presents. Not fun, but it helped. His dad asked us to buy him a new tv, and we said no. I think we were lucky to set the precedent this way. If he had said they had no food or a bill or something, it would be hard to say no. But he learned that we will say no.
But we know that eventually they will need something big, and we will have to use the money. So my (long-winded) advice to you is this. If you know her situation won't change, and knowing you can't change her, then start saving a little each month in an account she doesn't know about. And in the mean time, learn to tell her 'no' as much as possible so that she relies on you less. It is so much easier said than done, I know. But you aren't doing her any favors by rescuing her. I know you know this.
Ultimately, you have to decide in advance with hubby how much you can afford to help in the long run. And figure out in advance plans to help meet her needs. What programs or non-profits offer services that will be able to help her. The more you know ahead of time, the less you'll be throwing good money after bad because you weren't prepared. And also be strong about making sure you don't sacrifice your future by spending more than you can afford.
It's so tough, I know. I never thought I would be raising kids and dealing with this at the same time. Not ready for this.... :-( Good luck to you, and know you are not alone.