Narcissists only occasionally recognize their own Narcissism! Even if they do, they rarely care.
The Denial Show - 2010 Sister Wives - 1.57 was released on: USA: 5 October 2010
She was a religious sister and they consider themselves to be brides of Christ. They do not marry.
Short answer? You don't! You let them find out about your sister on their own (and they will, eventually). Continue your regular (hopefully good) relationship with your in-laws as you do now.
Seeing as your sister is not a boss in a video game, I'd say that you don't and that your sister probably isn't old enough to be an actual narcissist.
She was a sister. Sisters and nuns consider themselves to be brides of Christ. They do not marry.
Your mother's sister is your aunt.
Your mother's sister is your aunt.
Your mother's sister is your aunt. Your mother's mother is your grandmother.
Your mother's sister is your aunt.
Mother Teresa's sister would take care of her mother then her mother died and after the death of her mother, Mother Teresa's sister had died!
Your dad's sister in law that is your mother's real sister.
Your Mother's Sister Is Your Aunt.