Triangulation is another trick of Narcissistic Mothers.
It involves creating a situation where two or more people, e.g. siblings, do not relate directly to each other, but mostly relate via the mother. This leaves her at the centre of the web.
This triangulation gives the mother lots of power, of course, and means that she controls the flow of information, the interpretation of that information, and the nuances of it.
It also means that she gets lots of Narcissistic Supply from it because everybody is relating to her, rather than to each other and cutting her out of the loop.
It can be easy to put siblings into a triangulation situation. She has possibly already divided them by making one a Golden Child and the other a Scapegoat.
She'll say things like, "Oh, your sister was asking how you were. I told her you were fine, but that Jimmy's having trouble in school."
And you, well-trained, might say, "That's nice of her to ask after me - do tell her I was asking after her."
That's if it's benign.
It might be malicious of course. She might also say, "Sarah said you were looking fat". This of course will make you upset with Sarah, and drive a wedge between you. And in the meantime she might be telling Sarah nasty things that you ostensibly said, to finish the job.
The narcissistic mother wants her children isolated. There is strength in numbers, and she doesn't want you strong. Plus, if she's the only source of information, it makes her feel important which is food and drink to a narcissist.