If your S-I-L indeed has NPD, I also think that an intervention would not be helpful & might also make things worse. Those with NPD do not live in a world where they can be the primary problem & I don't think you, or a group of you, will convince them. I also agree that if you and other family see through her haze and understand it to some degree, it should help all of you cope. I appreciate your concern for the children, but they may do okay provided your brother has a healthier personality. I think this is a situation where the healthier members of the family help each other compensate. Good professional help (as needed) for them would be a more fruitful approach. Since I don't think you can change the S-I-L much, the better strategy is to limit the collateral damage I think. Good luck.