How should I talk to dementia, narcissistic MIL? She has been nasty over the years and it is hard not to get annoyed at her.

You say, " Should I tell her about her annoying narcissitic behavior. does it do any good at all to tell her. She does not believe or remember anything we say. I have learned to not to visit much we go once a week. She is is Assisted living but needs help."

There is no point in telling your MIL about her behavior if she's not going to acknowledge it or recall you mentioning it. If you go to visit her once a week and she gets nasty, just let her know you're leaving BECAUSE she's acting nasty. That ought to get the message across to her loud & clear.

If she's living in Assisted Living and 'needs help', pay extra to get her the help she needs, plain & simple. If she needs you to bring her supplies or something on that order, bring them along with you when you visit or have them sent to her via Amazon.

Good luck!

Happily she doesn't live with you. For my partner, who had a narcissistic Mom who made his entire life a misery, he called her once a week on Sunday a.m. very briefly, and when she finally died he breathed a huge sigh of relief and his Sunday was completely his own. It worked very well with him.
I wouldn't be in contact with someone who made all contacts with him or her unpleasant myself, and it would matter little to me that we shared blood or genetic markers.

Fortunately she does not live with us. My husband is actually worse at dealing with her sometimes. Should I tell her about her annoying narcissitic behavior. does it do any good at all to tell her. She does not believe or remember anything we say. I have learned to not to visit much we go once a week. She is is Assisted living but needs help.

Chalk it up to the desease, makes it better to deal with. And see less of her. She is your MIL your husbands mother. Its his responsibility to check on her.

Why talk to her at all?

Must you talk to her? Does she live with you? Is she in a care facility? What is your husband's involvement with her? He should take the lead in addressing her behavior. I would have your husband sit down with her and you, and tell your MIL that her narcissistic conduct is not acceptable and will no longer be tolerated. Sure she'll have an excuse, will probably get angry, and may not even remember the discussion, but it'll give you a way to address the problem, and allow you to feel that you have expressed your emotions. After that, if you don't want to talk to her don't.

Yes, it probably is the dementia, unless she's always been like that, but you don't have to accept that as an excuse for her behavior.