Hi, I'd like to help you with your question.
It is difficult to tell someone that they are a narcissist. I agree with you. You will most likely get an angry response or a denial.
In most cases, when someone has a personality disorder, it is almost impossible to get that person to understand what you are seeing. They cannot understand why there is anything wrong with their behavior. And if you try to confront them, they will see you as the problem.
That does not mean they will not ever see it or ever get better. But it is something they will have to see for themselves. In other words, they need to have insight into their own behavior in order to see that it needs changed.
Narcissists feel the world is about them. It's not that they don't want good things for others, they just feel they are first. They know more than everyone or they are the best at everything, etc.
What you can do about your sister's behavior is work on your response to her. Realize that she does not mean you harm, so try not to be angry or upset with her. Also, don't feel bad if you need to remove yourself from a situation with her when her behavior bothers you. You are not obligated to join in when she expresses her thoughts or acts out. Remember, you are not responsible for her and there is no reason you need to placate her or be manipulated.
There are also some books that will help you. The more you know, the better able you will be to deal with your sister's behavior. One book is called The Wizard of Oz and Other Narcissists: Coping with the One-Way Relationship in Work, Love, and Family by Eleanor D. Payson. Another is The Object of My Affection Is in My Reflection: Coping with Narcissists by Rokelle Lerner. You can find these on Amazon.com or your local library may have them for you.
Please let me know if I can help you with any other questions.