Seasons,
Is you SIL a person who likes to learn about what makes people tick?
If she is, maybe you could share an URL with the basic traits of narcissism, for example. You think that might help, or is it better left alone?
I understand the enormous desire so many of us have/have had for someone IN the family to see, along with us, the effects of N in the family. Sometimes, it just ain't to be. And only people outside the family system can give us support about that.
But it depends on the people. Your sister N sound pretty poisonous, and maybe she's too slick for your SIL to stand up to, you think? (After all, consider how many people here--smart and caring all--were deluded by Ns for literally decades upon decades. If you've only gotten your N-knowledge recently, might be hoping for too much to hope that your SIL would catch on as quickly. Plus, she's one family-step removed from your Sister N, so she's likely not quite as motivated to figure her out, as you are.)
((((((Seasons))))))
Hops