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?

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.

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?

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.

Come to terms by stop labelling her. You look at everything she does through the lens of laziness and lack of empathy. You put "too much trouble" in quotes and I am not sure that is her response or yours,

I can think of a number of reasons besides laziness. For instance, are you sure your sister is physical and mentally healthy. I have seen a lot of people who are really sick, won't admit their illness because they have been like that so long. Be empathetic to your sister. In her mind she is probably doing the best she can.

Whatever, the reason your sister is not performing up to your standards, seems to me a focus on whether your sister is putting your mother in an unsafe environment. If so, you could enlist help from senior protective services.

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?

I'm another one who is going to stick up for the sister a little bit here. I was the caregiver who was scrutinized by family/friends. Are there dishes in the sink? Well, yes, but I run the dishwasher every 2 days and those dishes are from last night's dinner and this morning - they will be washed tonight after dinner. It's very easy to see the laundry that's sitting, but difficult to see that I just did two loads and put it away while no one was looking. Sorry I didn't get all of it done. I had a family member make an unannounced visit and email me later with her list of things to either do by myself or mandate that a twenty-something person she specified must do them. How do I mandate someone? Seriously? Plus, elders frequently don't allow uninterrupted time to finish a big project (like scrubbing the floor), so sometimes those things get put on the back burner inadvertently. I had a million other things going on in my life at the time, but maybe no one knew that or cared. I was so exhausted that I prayed something would happen to me so that I could be in the hospital a few days. (It sounds crazy at the present moment, but that made sense at the time in that state of mind). To top it off, elder's house was in better shape than my own. When someone appears lazy and ineffective, sometimes it's exhaustion and feeling like life has become a bottomless pit. Sister may have simply had it up to here.

First thing that stood out to me was your statement "When I ask mom to ask her to help me out ..." Don't ask mom to intervene. YOU talk to sister directly with your concerns and complaints. Second thing, consider using part of mom's SS to hire a biweekly housekeeper. She/he could be scheduled to come a week or so (midway point) after you leave. Third thing, consider using a part of mom's SS to hire a personal health aide that can come to do grocery shopping, light meal prep, activities of daily living assistance, etc. They run $18-20/hr where I live. My mom has 30 minutes assistance with compression hose, dishes, medication reminder, shower assist, laundry, etc in the morning, and 30 minutes in the evening. You can talk to local companies - they don't have to come every day. Good luck. I suspect sis is burned out, depressed, and probably has some underlying health issues.

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.

I have a sibling that wouldn’t participate in active caregiving with our dad, he sat and stared at either TV or his phone. What he did do was complain loudly about what he was asked to do and put out on social media that he was a sole caregiver under tremendous stress. I also walked into a mess after his turns with dad, and though I didn’t say anything to dad, I could see my dad’s strain at the hostility he got from my sibling at being asked to do anything. For me, the answer was to stop expecting anything, not waste time feeling resentful, and find other ways to accomplish what needed to be done. Sometimes the people around us, for a myriad of reasons, just aren’t capable of doing what’s needed. And it’s okay to release both of you from expecting it to be different

In my opinion, katiekat2009 finally SAID it! "And sometimes people are just slobs." And lazy, too.

None of us here KNOW that the OP has been out of the picture 'doing nothing' all these years and is just now taking up her 'rightful place' as a caregiver. Maybe the sister WANTED to be the primary caregiver and get to live rent free, who knows, really?

Nor do we know that the sister is or is not lazy, but we DO know she's neglecting mom and is not doing any housekeeping by virtue of the fact that there are no housecleaning products in said house.

I clean my house. I can't do so without cleaning products, rags, bleach, paper towels, Lysol toilet bowl cleaner, Windex.............or even just a gallon of bleach. Which wouldn't clean the windows or the mirrors, but at least the toilets, tubs, sinks and showers would be clean.

Cookie, if you and your sister are going to share the duties of caring for mom and mom's house, it looks like you will be doing it virtually by yourself. For whatever reason, your sister is 'there' but not engaged in care giving, cleaning or cooking. This is the arrangement your mother has agreed to, for some reason. She doesn't want to talk to your sister about changing things, and who knows why, but maybe your sister isn't so nice to her if she complains. Ask mom what she would like YOU to do while you are there for your portion of the time. Would she like you to clean? Cook meals and freeze them? Help her with showers? Etc. Then go about doing whatever she would like you to do. Keep in mind that you can't scrub the house from top to bottom and expect it to STAY that way when you're gone, so don't have that be your goal. Just tidy it up to the point where it's livable for the extent of time YOU are there.

You can't change your sister. But you can change how you react to her and what she chooses to do or not do. Is it fair that you will be given the lion's share of cooking/cleaning & caregiving during your stint? Nope, it's not. But the main thing is that you KNOW your mom and her home will be properly cared for while YOU are in charge. While your sister is in charge, all bets are off.

It's really all you can do here. If you have a decent enough rapport with your sister, maybe you two can chat to see if you think she's depressed or burned out or why she seems to have no interest in anything these days. Best case scenario, the two of you forge a better relationship coming out of this than when you went into it.

Worst case scenario, nothing changes but you decide not to let your sister get under your skin, and to just do the best job YOU can do for MOM.

Wishing you the best of luck in a tough situation.