Hi Agnes1. I hope this blog finds you well.
From the perspective of a person with a mental health condition, I can tell you that if you have told her once and she doesn't listen, then she still won't listen no matter how many times you tell her and that's because she is tired, has little energy and still is not well enough to hear you. During my early diagnosis, just going to the letterbox was overwhelming so I get it. But I ate celery and carrots because I didn't want to have dishes, so my house wasn't in a terrible state like your sister's as you described.
I am not a counsellor so I am going on a ledge here but I think the reason your sister hoards and doesn't clean is because it is familiar and that makes her feel safe. Having your agency (power to act) taken away (even if it is only cleaning) can be as wonderful as it is frightening. You may have to whisk her up and take her to your house for lunch before you take any further steps (like going out for coffee and showering and wearing something clean and neat, which is something you can both do).
Whichever boundaries you choose (if you do), be kind, speak in a calm tone and never take your love away -be genuine, look after yourself and family and show her that you care. Our words are 7% effective in communication but our tonality is 38% effective. (Think of the word F*** - how many different tones there can be and how it changes the meaning of so many things).
In addition, sometimes writing a 'love letter' about all the things you love about your sister, to your sister, and some of the things you would like to ask her (without asking them in the letter) is a great way to be heard, even when it is hard to do.
I sincerely believe that language and words have enormous power and because of that, we have the duty to craft them carefully and the ability to craft them beautifully.
I hope this helps.
Helen262