How do you navigate difficult situations while remaining virtuous and not burn yourself out?

So, I have a sister who is autistic. She's not too bad in the grand scheme of things but she's just really mean-spirited and she doesn't really pick up on social cues. She's often in denial and she makes baseless and rude judgements about the people around her and believes it to be true even with zero evidence. She's an accountant and my mother and I don't necessarily have to worry about her financially; it's just everything else.

She doesn't have any friends. She doesn't clean her house even though she has a dog. She doesn't walk her dog and her dog pees and poos inside on a pad. She doesn't shower that often. Doesn't brush her teeth every day. Barely ever goes outside. Orders door-dash every meal, etc.

My mother and I team up to help her try to live a life with some semblance of dignity. My mother goes to clean her house twice a month and I try my best to be her friend.

However, at the end of the day, it's kind of thankless. Because she's autistic, she doesn't really make the connection that we're going out of our way and doing all this stuff for her. She still snaps at my mom for stuff, find her incessant and annoying, etc. For me, even though I am trying my best to be her only friend, she'll make baseless judgements about my or my girlfriend and behave in pretty rude ways. She believes without doubt that her perspective is always correct and nothing anyone says can convince her otherwise.

Dealing with her takes an emotional toll on my mother and I but we're trying our best because she is family and this has been going on for years.

Recently, some stuff happened and I got really mad at my sister; more than usual. Usually, I can just take it and rationalize that she isn't normal and healthy and I can't blame her because she isn't in control of what she's doing. However, it's like, if you get slapped in the face, even if you know that the other person isn't really at fault, the pain still hurts and it adds up.

I'm just not sure what I should be doing in the long-run.

Am I supposed to continue to see this as an emotional test for me and try to reach some sort of mental or emotional nirvana where nothing my sister says or do can hurt or bother me anymore and I can just continue to give and give and give and give and be okay with it?

Will I have to cut her off out of my life one day and then my stoic test will be how I deal with the guilt and inner turmoil of being a bad and selfish person?