we have a very similar situation here with my wifes sister.
the thing you have to remember is that your sister because she was raised by a narcissist may have never differentiated properly. She may have been forced to accept an alien false sense of self put forward by the narcissist as a projection of some aspect of himself. Since she finds it hard to live with the alien self she will project it back onto him and most likely try to controll him a bit so she can feel whole. She also has most likely thought about how in the world she can cope after he is gone and no one is there to take the projection and it scares her. I know it sounds SOOO crazy -- because IT IS CRAZY. But anyway when you criticize him she most likely feels you criticize her and if she has NPD she feels narcissistic injury. You didn't say what your mother was like as a caregiver but children often look to the father as some kind of an ideal to emulate. If you don't do allot of hard work as a child of a narcissist you end up with a really screwed up sense of right and wrong. often being "good" means putting up with the narcissists crap no matter what the costs.
being "bad" means being a strong happy independent individual who makes their own decisions.

also no doubt she is being played by the narcissist in some way most likely he is trying to emotionally blackmail you "i'm old and am going to die" "how could you do this to me after all i've done for you" etc etc you know the drill. he most likely sees her as a way to deliver the hurtfull message and since she still has the manipulation buttons -- bingo.

unfortunately so many children of narcissists end up narcissists themselves or even borderline. it's a sad state and you have my empathy that's for sure