How do I deal with anger towards my sister when I come to takeover Mom’s care and see that she has been neglected and the house is filthy?

Everyone has different ideas of clean and tidy. My mom's place, to me, is a hot mess, and realistically? It's really grubby and could use a good airing and scrub down, but it's not going to kill her to sit in a dusty, unorganized apartment.

I gave up some time ago trying to help her. Then she actually TOLD me to not touch or move her 'things'. I knew it bothered her when I took out the newspapers and tried to organize a little, but I didn't know it really was the problem to her that it was.

Now when I visit, I visit her in the common living room and don't even go into her apartment at all.

In regard to the housework, I could not tell from your post whether we are talking about a genuinely dangerous/unhealthy state of affairs, or simply about messy disarray. If the first, then obviously something needs to be done, but if the second, please keep in mind that, as another poster has put it, tolerance to mess varies a great deal among people. I would suggest not cleaning to your preferences, but to your mother's. Which might well reduce a lot of work and resentment and believe it or not, your mom might be happier too. I'm one of those people who find some degree of mess stimulating, but neat and tidy? Well so are graveyards. Like everyone is in their place... I try to be compassionate toward neat freaks, but sometimes I wonder if they really believe keeping this tight control will save them from illness, death, taxes, etc. If it is not actually a matter of safety, then why waste energy trying to control inanimate stuff?

How about having someone come in to clean? Is it normal cleaning or too much stuff, or a hoarder situation? Strip the place down to bare bones. No worries that workers coming will take anything, and the house is still comfortable.
I worked for a man whose family stripped his house down. He still had couches, chairs, dining table, but nothing there to take. Even his cooking utensils and things, were nothing anyone would want. He was still very comfortable, had an old TV, and could watch his videos, and they didn't have to worry. He still had everything he needed.
Not a lot of things to get dirty other than dishes or maybe crumbs. Easier to clean.
You could try that.
Or get some help with a carer. Maybe they can be buffer between you and sis and do some light cleaning. Keep it up, do a load of wash.
Or move her to a small apartment. Everything on 1 floor. Less to keep up. I know when my parents health went, the house seemed to need new everything. Even a roof!
That way you can help with mom but not be overwhelmed with care of a house if she is in one. And you can stop fighting with sis. If you cont to fight it will probably get worse. Eliminate the problem. It does you know good to get upset. She proved she will do bare minimum. Get it done now. You dont want situation to get worse.

There's always going to be the one who will get away with what they can. Takes me back to the days of my sister and I sharing a bathroom. Her half of the counter a pig sty of cosmetics, samples, powder dust; she'd take a shower and go to bed, not washing the tub; I preferred a bath and would have to clean it if I didn't drag her sorry ass out of bed to get back in there and clean it. Simple consideration. There seems to be some important facts neglected to be mentioned here...
Is your sister dealing with any healthy or physical limitations? Can outside help come in to help with your mother's physical care? How are meals being provided to your mother and by whom? What does your sister do all day besides watch TV and your mother?
I would not put your mother into the middle of it. However I also sadly have no answer for how to come to terms with it, unless you kick your sister out of the picture and document your time and efforts and /or hire someone so you get a reprieve/respite. If you are doing more than your fair share, maybe you might want to invest in a consult with a certified elder law attorney and see about creating a caregiver agreement so that somehow you are compensated for your efforts, and your sister, significantly less considering her lack of efforts.

Don't ask your mom to ask your sister to help. You talk to your sister directly. You don't have to come to terms with her laziness and lack of empathy for your mom. If your mother gets angry when you even mention the conditions of the home and of herself, then don't press the matter with her. Clearly it doesn't bother your mother if she lives in filth and squalor or if she's dirty and unhygienic herself. Don't let it bother you either. It seems to me that in your situation, your mother and sister both expect the chores and work to be done by you when it's your turn to be there, and that they don't mind waiting until you get there. Clearly neither of them has the slightest respect for you. Have a talk with both mom and your sister and let them know that you're not going to come for your caregiving time every two weeks and spend it cleaning and doing everything your sister refuses to. Let them both know that you will bring in outside help to do your time at mom's house because you will not stay at her place if it's going to be a disgusting mess. They'll come around.

Is this new behavior for your sister? Do you think your sister is ill? On drugs? Drinking? If this is new behavior it sounds like something has happened to her. Does sister bathe, wear fresh clothing?Gaining or losing weight? Does she live with mom?Has she been to the doctor herself?

Call your area agency on aging. Ask for a needs assessment for your mom. Find out what services are available. Is it your mom that gets angry or sister that gets angry? I would get some scales in the house and weigh your mom each time you come in and when you leave. what are your moms health issues that she needs care? Is she taking her meds?
I know it’s discouraging but something must be going on with your sister to be so passive aggressive. Does she speak to you, is she animated? Does she leave the home when you come in?
I’m missing something. Your mom must have eaten in three weeks. Who manages moms finances? There must be more to this story.
Regardless of the reason, you are understandably upset and I’m sorry. If this isn’t what you agreed to, you need to ask sister what her intentions are. If she lives with your mom and mom wants her there then that’s harder as your mom may be afraid to be alone. Your sister may be burned out if she has been there a long time and your coming in is recent.
I would try to have a talk with your sister and find out what is wrong. If this is normal for her, then you may have to renegotiate what you are willing to do.

Why does your mom get angry when you ask her to ask your sister to help with all the chores you are doing?

My gosh, if my sister came here every 2-3 weeks to give me a break, I would be so damn thankful. Kuddos to you for stepping up to help!

Still, I understand about your frustration with the cleaning. How about you stop doing it too and hire a cleaning person on mom's dime? Take mom out to lunch while the cleaners are doing their thing, maybe she will like that idea? Maybe sister will too?

Some people are just a lot more comfortable living in disarray than others, I've got one of those in my family too. It's only natural to feel resentful when you have to spend your time cleaning up after her but that's not solving anything. If you need this arrangement to continue and your sister's contribution ends at being physically present then one solution is to hire a cleaner (mom pays for this, not you) and to look for alternatives for meals for your mother if she needs that, perhaps meals on wheels? What kind of personal needs are being neglected?

My honest opinion is that a new arrangement may be needed, sooner rather than later.

Turn up to help & have all that too?...

Resentment?
Is that what you feel?(understandable imo)
Dictionary says resentment: bitter indignation at having been treated unfairly.

But you're right - you can't change your sister.

So what to do? Keep doing your care your way & letting her care her way?

Or get a whole new plan.