What do you think about me refusing to be alone with my youngest sister, despite the pain it gives my parents?
Oh, I don't know. Why don't you want to be around her alone?
It makes me feel uncomfortable?
You should not avoid family and hurt your parents. If you have a problem with her you should explain to them why.
What if I don't want to feel uncomfortable around my sister?
If you are uncomfortable around them, you should explain that to them. They will respect you much better if you are honest with them about your problems with your sister.