Okay my bad for misunderstanding your post. But you're right, it's my own very similar experience that allows me to easily spot it and identify it.
The sister's behaviors are textbook examples of narcissistic personality disorders.
She is even trying to control communication between other people, as if they are her property and not individuals.
That's why I suggested lying. It can even be lying by omission. Only good things can happen by putting the sister on an information diet, because what else can you do with a narcissist? You can't change them, you can't reason with them, you can't openly talk with them, all the while they behave unreasonably sour and make unreasonable demands.
Personally, I would cut them out of my life. But the next best thing is letting them know as little about your life as possible.