The nice thing about being a grown-up is that you don't have to rationalize your choices to anyone. This includes your family. Just because you are related to someone does not mean you are obligated to sacrifice your happiness for their sake. You can still be a responsible family member by contributing financially to your mother's care, calling regularly to see how she is, and visiting once a year (for a short period of time, and staying in a hotel so you don't have to absorb all the dysfunction and feel suffocated while you visit). It doesn't sound like you really want to cut your family out of your life---you just need to maintain distance for your own sanity. So do so.
You don't need to try to rationalize with them---just go and live your life. Don't get drawn into verbal or physical altercations with your sister--tell her that you have made your choices and that is that. If you don't explain or rationalize, there can't be an argument. Arguments happen when a person tries to explain themselves, and the other person refutes , contradicts, or dismisses the point. It's circular, and no good can come of it.
You have been in your family long enough to know that the dynamic and people aren't going to change or be different, no matter how much you would like it to be otherwise. You know that you don't get along with your mother and her presence makes you mental, and that your sister doesn't understand you or your lifestyle. That won't change. So give up the hope that this will all disappear and that you will all get along one day. Deal with what the reality is, and go about your life accordingly. Total abandonment of your family ties isn't necessary, but maintaining your sanity is. And there ain't nothing wrong with that.
Don't feel guilty. Contribute financially to your mom's care so the burden doesn't fall entirely on your sis, call mom once in awhile to see how she is, or send a little gift now and again so that she knows you are thinking of her. This way, you are still maintaining family ties, and letting your mom know you still care---but doing so from a distance, where your life choices aren't being scrutinized and you aren't feeling overwhelmed with the dysfunction within your family. And then feel good about it all, because you are a grown-up, and have made a responsible choice without compromising your own feelings of well-being.
Remember--no guilt! Guilt is just a waste of emotion and energy. Good luck!

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