Thank you for writing in and sharing it
I understand being with narcisstic in-laws must be challenging for her, as they are self-centered need to be the center of attention all the time, and behavior can disrupt family .lacks empathy, self-importance, believe them as special, is often envious of others. Don't try to confront partner, it may lead to a huge fight where partner defends the parents, rather tell her to mention to her partner in a subtle way. not mentioning that inlaws are narcissistic.
tel her not to take insults personally,
Try to label certain behavior as narcissistic, this will make easier on your sister when they insult her,
Accepting their behavior, yes it is not easy to accept such behavior.
she might find it it is better to ventilate at that moment with the partner, but it may lead to a further fight.
please tell her to connect and share in a chat session.