Unfortunately I have been dealing with a very similar situation with my sister. We have gotten her to agree to a budget. The budget gives her some discretionary money and we pay her other bills like rent, utilities, cell phone. The problem we have is that her savings will not be sufficient for her estimated lifespan and she spends her discretionary amount foolishly.
Perhaps you can get her to agree to a set discretionary amount and then put limits in place like credit card limits or use a debit card that doesn't have overdraft.
My sister, is always asking for more money from the savings.we are going to try to tell her what an annuity will pay per month and get her to accept that she can’t have more than the annuity will pay. Sort of like, this is your paycheck for the rest of your life, make it work.
In the end, do the best you can and don’t stress over it. Some people simply can’t live within their means.