One common tactic used by narcissists is
triangulation. From a psychological and communicative perspective, triangulation occurs when family member A has an issue or conflict with family member B, and rather than address that issue directly with family member B, family member A brings in another person, family member C, to discuss the issue (hence creating a triangle of communication). The concept of triangulation is well-known within the study of dysfunctional family systems because when the third family member is brought into the situation, the narcissist typically does so as a way to paint a negative picture of the person with whom he or she has an issue with. The narcissist does this in order to "split"the family and play the third family member against the one he or she is upset with. Here's the kicker though--the narcissist will not only bring a third family member in to talk about the issue with--he or she will also engage in a character assassination of the family member they have an issue with, thus creating a situation where family members B and C don't communicate directly with one another. Rather, they communicate solely through the narcissist, which is exactly how the narcissist wants it. If the two family members involved in triangular communication with the narcissist don't communicate directly with each other, then they can't ever verify whether or not what she (in this case, the narcissistic mother-in-law), is saying is true.
In families headed by narcissistic parents, poor relationships between the children is common. Triangular communication created by a narcissistic parent is especially damaging because no one would ever question the accuracy or truthfulness of a mother's story. And the narcissistic mother knows this! So stories are spread to the other children, stories that paint the scapegoated child in a negative light. When the other children--the scapegoat's siblings--hear these stories from the parent, it infuriates them that their sibling would treat their poor mother in such a way.
This is exactly what happened to my husband Aaron. He has a terrible relationship with his siblings due to his mother's triangular communication. His mother has engendered a negative view of him in the eyes of his father and his siblings in an attempt to build contempt for him. And boy has it worked. When he attempted to bring this to their attention, they accused him of being ungrateful and jumped to this poor woman's defense. She used triangular communication to drive a wedge between her children--and she continues to utilize this tactic to keep them apart.