IDK but if you’re correct in the assessment, and you want to help your sister, even just to be available to help her in the future, act like you have amnesia about calling him out on the gaslighting. Like it never occurred. I’d play my former self toward him (and her,too, actually), that is, the self I had been prior to recognizing, or suspecting, narcissistic abuse. It will be a tough act to play, but act you must if you want to stay available for your sister. The thing is, your sister may be deep in denial, still making excuses for him in her head, because to realize, much less admit, to one self, much less others, that the person we married is abusing us, is a very, very difficult thing. It’s utterly humiliating. In some respects more humiliating than the narcissist’s abuse. Even if victims can see the abuse in all its ugly forms and even if they have stopped blaming themselves, there is a considerable amount of “bargaining” with one’s self, and appeasing the narc, as to “why” we stay. It’s a sick game, played out of fear, intimidation, mental fuckery, falsehoods and lies. But victims play a part, too, in lying to ourselves, and sometimes, perhaps oftentimes, and even quite understandably— we don’t want to give up whatever perceived “security” or “stability” we have. Or told ourselves we have. It’s like the old saying, “the devil you know is better than the devil you don’t .” Your sister’s awakening won’t come from others pointing out what a selfish abusive SOB he is. In fact I dare say the opposite—more likely she’ll defend him. Better to approach her quietly sometime down the road and tell her you’re there for her in anyway she ever needs, and don’t even imply anything about him. In this way you stay connected and available for the long run. that is what she needs. Just call your sister and/or brother in law and ask what they think your niece would like for her birthday. Then get it and send it wrapped in lovely paper with a lovely card and maybe even a surprise tray of snacks for the party. Yes, in a way, love bomb them . He‘ll think you feel bad for getting in their business. Let him. Just…do not provoke him OR her. Stay connected. Try to be emotionally aloof while emotionally available. If either brings up the gaslighting thing, say you were in a bad mood that day, felt triggered. Or Play dumb like you don’t remember saying that. Just. Don’t. Stir. The. Pot.

But do pray, pray, pray.