I’m sorry this message won’t get to you in time for Christmas. I’m responding any way because your sister visits you regularly and every visit puts you in the same position.
You owe it to yourself to tell your parents what happened. My guess is that they will not want to confront your sister about what she did to you because they won’t want to lose contact with their grandchildren. I hope they will, none the less, be supportive of you so that you and they can figure out to make her visits less painful for you. You could confront her for the sake of your own dignity, but it’s important that you know going in that she is unlikely to acknowledge that she did anything wrong. Narcissistic people rarely see any reason to change.
You have a right to take care of yourself. One way to cope is to stay with friends when your sister comes. If you can afford it, treat yourself to a mini-vacation. If those options aren’t available, stay out of the house as much as you can. Go to the library or a movie, sign up for extra shifts at work or do some volunteering — anything that removes you from the situation. There is no rule that you have to be a participant in her visits.
If you have the energy for it, an alternative way to cope is to focus on those kids. Don’t underestimate the importance to their psychological well-being of having a loving aunt who takes the time to listen to them, read to them or take them to interesting places. Going for a hike or to visit a local children’s museum will get you all out of the situation and will give the kids intellectual stimulation they don’t get at home. Introducing them to the world of books and to music and art will give them ways to separate from their mother’s lack of interest even when they are home.
People who provide an alternative reality for neglected and abused kids are called “benevolent witnesses.” Their interest and involvement can give kids a beacon for a better future. Many survivors of abuse have told therapists and researchers that what got them through was the occasional visit to a loving grandparent or the positive involvement of an adult friend.
Meanwhile — I hope you have some good friends and a positive direction for your own life. You are right to hate your sister’s selfishness but if that hate consumes your life, she is still hurting you. If that’s the case, I hope you will consider some therapy to help you get all the way past it.
I wish you well.