How do I deal with my angry mom regarding her choices and finances while she is getting worse and worse with dementia?

You say mom is living in Assisted Living.
Who bought a house for who and why?
Mom can't live on her own.
Mom, if she has been diagnosed as incompetent (has she)??? She can not make a purchase like that.

You can not ask someone with dementia if they want to move, if they like a particular place, or any other decision that has or may have long term consequences. Sure you can ask a person with dementia if they want a Tuna Sandwich or Chicken salad but you certainly don's ask them if they like an apartment in a AL facility, or if they want to buy a house.

You as POA may be skating on some pretty thin ice when it comes to financial decisions.
Frankly your sister may also be skating on thin ice when it comes to making decisions where mom is to be living. She should be in Memory Care not AL or heaven forbid living in a house by herself.

Suggestion:
Sell the house that you bought.
Find a Memory Care facility
Use the funds from the sale of the house to pay for her care.

She "forgets" the mean things she says cuz she has dementia.
She repeats things cuz she has dementia
She texts the same things over and over cuz she has dementia

Please read Alva's advice and then reread it.

"The house is hers, and in her name."

Read your POA, is it immediate or do you need a Doctor to declare Mom incompetent to make informed decisions. If immediate you can step in and make decisions for her. If you have not gotten a formal diagnosis of Dementia, I would do it. Its important to know what type your dealing with. Has a lot to do with what medications can be used on what type of Dementia. You will need a formal diagnosis anyway if your POA requires it.

Realize now that Mom has short-term memory loss. She has lost the ability to reason and show empathy. You are now the adult and she is the child. You need to do what is best for her and you. Right now an AL is best for her. Probably Memory Care in the future. Sell the house. Your POA should give you that ability. Sorry, Mom can no longer make decisions concerning her life. It's now you. Toughen up. I did what I needed to do and I refuse to feel any guilt for those decisions, right or wrong.

She has dementia or she does not.
You are acting for her financially or you are not.
If you bought a home for her it had better be in her name deed and title, and I think buying a home for anyone of this age is very poor decision making on the part of a POA who is RESPONSIBLE for someone with dementia.
SEE AN ATTORNEY AT ONCE with all the details; this sounds like fraud and abuse.
You say she is pissed "we bought a house". WHO BOUGHT A HOUSE? Because if it isn't your MOM who bought this house herself, then you could be in very deep trouble.
Either your mother is competent and in charge of her OWN finances, or she is NOT, and YOU are responsible and handling her finances as her POA.
WHICH is it?

You, as POA are held to the highest standard of LEGAL fiduciary care. You are responsible to act in your mother's best interests if she is not competent to act for herself. How in the world is buying a home at this age in your mother's best interests? Can you tell me that?

YOU need to see an Elder Law Attorney at once, because I believe that your mother has a very good case for elder abuse and fraud given what you say. This home deed and title had BETTER be in her name ONLY. Otherwise I am afraid you badly need an attorney.

Your mother had an operating POA. That means she was not competent to make financial decisions. It cannot go both ways. She cannot be in control of her life, finances and purchases one moment, and YOU in charge of the the next. And there is no way on earth that buying a home at this point is in your mother's best interests.

The POA should not have LET HER make decisions regarding buying a home knowing she has dementia. You are held to know the law. If you do not your POA pays for an attorney to explain the law to you. I do not believe given the details you give us that this was lawful. Get to an attorney ASAP.
And if this house is not in your MOTHER'S NAME title and deed, then your mother is correct. This would be fraud and elder abuse. Someone could end up in the slammer on this one.

SEE AN ATTORNEY AT ONCE with all the details of this.
This is a complicated mess, and it is way beyond what any Forum can advise you on. You need an attorney. If you are POA your POA pays for expert advise. That doesn't mean a Forum full of folks from around the world. It means attorney.
Good luck.

Her boyfriend started this addiction issue you say? Is he still licensed? He violated 2 ethical issues that can be reported to the state. The first is he should not be writing scripts to someone who is close to him and not being seen as a patient. The second is that the state should do a forensic review of the amount of scripts he wrote for your mom. This is done by filing a complaint to the state physician licensing board. He is retired and holding the " loaded gun".
I am also questioning how you and your sister dealt with the detox. Can you fill in those missing pieces? She should still be considered an addict so I am not surprised she is angry. I also should ask is why AL? She is not locked down and can still get access to opiates. She may not be safe yet

Your mother has lost the ability to reason, so these conversations will only frustrate you both. Please stop having or participating in them at all. She once decided to name you and your sister as her POA’s for financial and healthcare decisions for a time when she couldn’t reliably do so. That time is now. Make the decisions needed, without mom’s unrealistic and unreliable input. Realize that unfortunately, whatever “happy” was for her is likely over now. Take her to her doctor for a “checkup” but forewarn them about her out of control anger and aggression. There are likely meds to help calm her behavior. It’s hard to accept when the person we knew is so changed and not someone we recognize as well. Remember mom trusted you and knew you’d be in her corner for a time such as now. I wish you both peace

I'm not sure why you and your sister would decide to buy any house for a woman who obviously can no longer live by herself. And is a drug addict at that.
Perhaps now it's best to sell the house and use the money to place her in an assisted living facility or memory care, as she is only going to get worse and will require much more care.

I’m so sorry you are going through this. I dealt with very similar issues when my mom, aged 84, developed Alzheimer’s. She also had cancer, had been an alcoholic, and was very angry with me. One day she ran me down with her walker and hit me. Please realize that this is the disease process and not the mom you once knew. She needs memory care, so my advice is to find a place with a low staff- to-patient ratio (4-6) trained in dementia care. It’s expensive but well worth it. They know how to work with these people and keep them busy. Good memory care facilities also have 24-hour medical staff that handle physical needs and medication. Most of the residents have comorbidities that accompany their dementia.
There is an excellent book that helped me through my adventure: The 36-Hour Day. It explains a lot and offers guidance on navigating this disease.
Currently, I’m helping my 90-year-old dad who has vascular dementia.
You will feel guilty no matter what you do. Your mom is not well and therefore angry. All you can do is make the best decisions you can for her to keep her safe and as healthy as possible. You cannot make her happy. She is so lucky to have you!

Trying to reason with someone with deficits to their reasoning ability is painful, frustrating & futile.

Does Mom have a diagnosis of dementia or cognitive impairment?

While a label of dementia or impairement can be just that, a label.. it may help you to mentally step back from trying to reason with Mom - and fully into the role of doing what you need to do (hard as that is). Either with much less discussion, or with a simple statement of fact. Eg Mom, you live here now.

I have seen the hardship all round when a family member tells me their LO has some dementia or impairement yet is also expecting their LO to make rational decisions. They tell their LO somethinh over & over (or write signs) but their LO lacks the reasoning ability to understand.

Why in the world would you buy a house for someone that is not mentally competent? Makes zero sense to me and an addict to boot.

You two need legal advice so you do not make another bad decision.

In the meantime stop discussing this with her, her brain is broken, start planning.

Try and take the emotions out of it, this is business someone needs to make sound decisions for her, if you are the DPOA then it is you. You can move her to AL or MC and put her house up for sale using the funds for her care.

It is no longer about what she wants it is about what she needs.

Sending support your way.

Sell her house. Why does she even need to know it’s sold? If she’s unable to get herself there - she’ll never know it happened. She can’t be in charge anymore - she is not well. You and your sister fighting with her is only going to make you unwell. Do what needs to be done without worrying about making her mad. She’s already mad. Madder?? What difference does it make it she’s madder. She doesn’t need to know the details because she cannot understand or remember them anyway. You’re trying to reason with her or explain the logic to her when she has no capacity to understand it. Dementia goes one way. She will not improve. Her ability to understand the details will not improve. I would quit trying to include her in the decisions. She gave you the POA to make decisions when the time came. The time has come. It feels weird to be less than truthful but the rules are out the window. No point in saying anything at all she doesn’t agree with. Perhaps you can focus on getting her involved more in her AL or memory care community by being present there and going with her to activities until she’s comfortable doing them on her own. Out of sight is real with dementia. Focus on what her life is now .. not what happened before or how you got to here. Fix all that in the background while doing what you can to be present in what her world will be now, which is hopefully memory care.