My mother is 91 and very competent. However she is financially irresponsible. How can we get control of her finances for her care?

Sounds like a circle of codependents here, why in the world would your nephew pay her bills? Hopefully he had enough sense to make a claim to the POA for his money spent on her.

Is your sister on her checking account? If not, get that done like now. Someone needs access to her money to be used for her.

Tell mom that she needs to pay for in home care, if she refuses then she will need to be placed in a home, these are her two options, set your boundaries, and stick to them. She is manipulating all of you.

She is managing you all by FOG, Fear=Obligation=Guilt, time to stop the cycle and get back in the driver seat. My best!

Why do you think she is competent? Sounds like she can't understand her financial situation like a business person would. That's not competent. Get her evaluated by a geriatic doctor or psych for competence and before the appointment tell him or her what is going on. Y'all need to stop paying her bills for her. That will be a wake up call if she is really competent.

Your nephew ought not to have paid off all her bills with his own money. That just means that your mother now owes her grandson that money. You, you the family I mean, had better get all of these transactions documented and straightened out before things go haywire and misty and hard to track, or that lad will be seriously out of pocket.

You say your mother has "all her wits about her." Really? What sort of person who has all her wits about her does not understand that she must pay her bills? Is she a fraud, is she almost psychopathically manipulative, or are you calling her stupid? Because if it isn't one of those unpleasant things, then you are saying that she cannot understand the significance and operation of money, which does not equate to having all her wits about her. It means that the financial wits, at least, have gone missing.

What sort of POA does your sister have?

As a first step can your nephew not make a claim against her for all the debts he paid - at least that would put some of it into someone else's care. Then could you arrange a meeting with her, her Dr and an accountant who specialises in elder management and let them draw up a conclusion having met her and discussed her needs and her monies. As a last straw if she refuses to pay for care where she is - which ever of you she is living with - tell her she will have to go into a facility because sister cannot manage. - you are in the fortunate position of having money, so many on here are without any assets. She needs to pay for help or pay a facility (and make sure she signs the papers). Tough love needed.

I'm trying to wrap my head around the practical realities of this - if she is truly bed bound and sister works who is caring for her during the day? How is she getting meals? How is she going to the bathroom? How is she gaining access to her credit cards and managing to blow her money with frivolous shopping sprees?

I wonder if you all have been trained throughout your lifetimes to jump when she barks, because the person who hold all the power her is not your mother, she would be very wretched indeed if your sister chose to play hardball and cut off her access to the phone and internet. And no, that wouldn't be abusive so long as eyes were on her throughout the day (human or virtual) and her basic needs were being met.

Why in the world would your nephew pay off her bills with his own money when it appears she has the income. At the very least I'm sure your nephew repaid himself after the home was sold. I would suggest contacting the local social service agency or a state social worker and get them involved. Maybe an outside authority telling her what she needs to do will help getting her compliant. at the least, they will see she is a vulnerable adult in need of care.

You say that your mother-in-law is very competent. Then you say she is bedridden and is using money completely inappropriately, is not paying her own bills (why in the WORLD does the Nephew pay her bills out of his money?).
So I am begging to disagree with the diagnosis of competent and I think it may be time for the woman to be examined by doctors for competency.
Dependent on the outcome of these exams it is either a walk away moment (should she be adjudged competent but still refusing to pay for her own care) or a moment to seek guardianship, placement, and management of her finances for her own care.

The Nephew being an "executor" will not figure in anything until this lady dies. The Sister she lives with, being POA is what interests me, as you say that the Mother is refusing to pay her bills, and etc. Generally it is the POA who handles the bill paying for the person, though of course if this woman IS competent she certainly can direct the POA to obey her wishes.
IF the Mother is refusing to pay any of her own bills I would myself not serve as her POA. I would be aiding and abetting fraud by a person taking services and refusing to pay for them. And she certainly would not be living with me. She would be in care, and whatever happens with her and her bills after that point, if she is in fact competent, is on her.
You can only do so much. But the short and sweet answer is that if she is competent you cannot do anything about what she does about her money. Sadly, someone paying her bills for her means she has no consequences of her actions.

Your nephew needs to be reimbursed for the bills he paid off for your mother with his own money.

You are a good sister for being so concerned for your sister. Your mother needs to get out of your sister's house, and it sounds like she needs a skilled nursing facility.

Will she qualify for Medicaid once the money from the house sale is used up? What is your mother now spending it on?

Please consult an elder attorney. The sticky part here is that your mother is apparently still of sound mind.

Are you sure the sale of the home money is not supposed to go into the trust?

First - from what I am reading your Mother IS NOT "91 and Very Competent". No one in major debt and physically not capable of taking care of themselves is truly "competent". The fact that she cannot understand and accept that she is responsible for her own bills (not your sister or your nephew's responsibility) is a major indicator to her lack of compentiency. You say your sister has POA. Is this a durable POA? If so, your sister already has control of your Mothers money. See the attorney who drew up the POA and get clarification on just what it says. Does your sister have rights to sign checks for mothers bills. If not this should address immediately.

As much as we love our elder parent(s). we must also see them for who they actually are at the advanced age of 91 and what they are still capable of doing. Help your sister find a good AL in your or her area. Or higher a good In Home Care Agency. You could end up burying your sister before your mother as this stress, etc. is dangerous (ask me - as I finally found out just how it has affected me and I am now doing what I need to do for myself). And yes My Mother did not completely understand just how her behavior affected me. She did have dementia. Which is something you can address with her primary care, along with her current behavior and medical needs. Realizing dementia is very sneaky and is never obvious in the beginning. Getting it diagnosed is not that difficult with assistance of her primary care.

Take each step in getting under control with love in your heart, determination to provide the best for your mother, but accept it may take some serious changes for all of you. Hugs, love and prayers for each small successful step in moving forward.
Mom will probably fight you all the way on these things. Talk with Sis and present a UNITED front with your mother. She may try keeping you (and nephew) divided in order to get her way. Stay UNITED and it will be easier for each of you.