There are some relationships that are just toxic, or at least unfixable. Because of your situation, you have to have some dealings with your sister. I suggest limiting contact with her to when it's necessary and always be polite and civil at these times. Also be polite and civil with her when together at the homes of relatives. Basically, at those times say hi, and go on to talk to others at the gathering. If you don't like to talk much at such times, just go off to listen to others, and don't spend any time socializing with your sister, since she stresses you out so much.
I get along best with my family when I am not with them, but have to occasionally spend time with them. Being polite and civil, and saying hi and moving on to be with others at family gatherings works best. It helped in dealing with my sister-in-law, and I had some serious reasons to dislike her. My keeping polite and civil kept the peace at all the family gatherings where we were both present. No need in her case anymore, as she passed away a couple of years ago.
Just remember, keep contact to a minimum, and be polite and civil at those times.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau