Hi Benguiat
re your comment:-
"However I still think deep down inside her there is a kernel of a rational person. And if I could express myself lovingly and clearly in a letter then it would give her time to absorb what I say and be persuaded by my arguments".
I do not mean the following comment at all unkindly but I did smile wryly to myself when I read the above. My ILs and BIL are all narcissists and I can tell you now that none of them at all rational. My BIL has turned spite agsinst us into an artform. ILs appear plausible to those int outside world and see themselves as good dutiful people. For the state of my mental health I keep my distance from them; its the only way.
In your case any letter will be seized upon by them and used to bite you on the behind. Don't send anything to them, best thing by far you can do is to cut them off. If my ILs were actually my parents I would have cut them out many years before now.
You may find the following from halycon.com helpful:-
"First, narcissists lack empathy, so they don't know what you want or like and, evidently, they don't care either; second, they think their opinions are better and more important than anyone else's, so they'll give you what they think you ought to want, regardless of what you may have said when asked what you wanted for your birthday; third, they're stingy and will give as gifts stuff that's just lying around their house, such as possessions that they no longer have any use for, or in really choice instances return to you something that was yours in the first place. In fact, as a practical matter, the surest way NOT to get what you want from a narcissist is to ask for it; your chances are better if you just keep quiet, because every now and then the narcissist will hit on the right thing by random accident.
It's very hard to have a simple, uncomplicated good time with a narcissist. Except for odd spells of heady euphoria unrelated to anything you can see, their affective range is mediocre-fake-normal to hell-on-Earth. They will sometimes lie low and be quiet, actually passive and dependent -- this is as good as it gets with narcissists. They are incapable of loving conduct towards anyone or anything, so they do not have the capacity for simple pleasure, beyond the satisfaction of bodily needs. There is only one way to please a narcissist (and it won't please you): that is to indulge their every whim, cater to their tiniest impulses, bend to their views on every little thing. There's only one way to get decent treatment from narcissists: keep your distance. They can be pretty nice, even charming, flirtatious, and seductive, to strangers, and will flatter you shamelessly if they want something from you. When you attempt to get close to them in a normal way, they feel you are putting emotional pressure on them and they withdraw because you're too demanding. They can be positively fawning and solicitous as long as they're afraid of you, which is not most people's idea of a real fun relationship.
I always have the problem that I get fed up and stay away from THEM long enough to forget exactly what the trouble was, then they come around again, and every narcissist I've known actually was quite lovable about half the time so I try it again. A clue: Run for cover when they start acting normal, maybe expressing a becoming self-doubt or even acknowledging some little fault of their own, such as saying they now realize that they haven't treated you right or that they took advantage of you before. They're just softening you up for something really nasty. These people are geniuses of "Come closer so I can slap you." Except that's not the way they think about it, if they think about it no, they're thinking, "Well, maybe you do really care about me, and, if you really care about me, then maybe you'll help me with this," only by "help" they mean do the whole thing, take total responsibility for it, including protecting and defending them and cleaning up the mess they've already made of it (which they will neglect to fill you in on because they haven't really been paying attention, have they, so how would they know??). They will not have considered for one second how much of your time it will take, how much trouble it may get you into in their behalf, that they will owe you BIG for this no, you're just going to do it all out of the goodness of your heart, which they are delighted to exploit yet again, and your virtue will be its own reward: it's supposed to just tickle you pink to be offered this generous opportunity of showing how much you love them and/or how lucky you are to be the servant of such a luminous personage. No lie they think other people do stuff for the same reason they do: to show off, to perform for an audience. That's one of the reasons they make outrageous demands, put you on the spot and create scenes in public: they're being generous they're trying to share the spotlight with you by giving you the chance to show off how absolutely stunningly devoted-to-them you are. It means that they love you; that's why they're hurt and bewildered when you angrily reject this invitation.
Appearances are all there is with narcissists and their self-hatred knows no bounds. The most dramatic example I can think of is from John Cheever's journals. Throughout his life he had pursued surreptitious homosexual activities, being transiently infatuated with young men who reminded him of himself in his youth, while also living in a superficially settled way as a married family man, a respected writer with an enviable suburban life, breeding pedigreed dogs and serving on the vestry of the Episcopal church. When his secret life (going to New York City for a few days every now and then to pick up sailors and other beautiful boys for brief flings) came to scandalous light, his family sought to reassure him by telling him that they'd known about his homosexual activities for years. Now, a normal person would be ashamed and embarrassed but also relieved and grateful that scandal, not to mention chronic emotional and marital infidelity, had not caused his wife and children to reject and abandon him but not the narcissist! Oh, no, Cheever was enraged that they would ever have thought such a thing of him -- if they really loved him, they'd have bought his artificial "country squire" persona: they would have seen him as he wished to be seen: they would have believed his lies without question or doubt.
Narcissists don't volunteer the usual personal information about themselves, so they may seem secretive or perhaps unusually reserved or very jealous of their privacy. All these things are true, but with the special narcissistic twist that, first, their real life isn't interesting to them so it doesn't occur to them that it would be interesting to anyone else and, second, since they have not yet been transfigured into the Star of the Universe, they're ashamed of their real life. They feel that their jobs, their friends and families, their homes and possessions aren't good enough for them, they deserve better.
Narcissists not only don't recognize the feelings and autonomy of others, they don't recognize their own feelings as their own. Their feelings are sort of like the weather, atmospheric, acts of God. The narcissistic think that everyone's having the same feeling as they are. This means that usually their own pain means nothing to them beyond the physical discomfort it has no affective component. When they do get some painful affect, they think that God is punishing them they think that their trivial errors are worth God's specific attention to their punishment. If you try to straighten them out, by telling them that your feelings are different, beware: their idea of sharing their feelings is to do or say something that makes you feel the way they're feeling and, as they make a point of not sharing anything desirable, you can expect something really nasty. The sad fact seems to be that narcissists feel just as bad about themselves as they make others feel about them.
Narcissists are noted for their negative, pessimistic, cynical, or gloomy outlook on life. Sarcasm seems to be a narcissistic specialty, not to mention spite. Lacking love and pleasure, they don't have a good reason for anything they do and they think everyone else is just like them, except they're honest and the rest of us are hypocrites. Nothing real is ever perfect enough to satisfy them, so are they are constantly complaining and criticizing -- to the point of verbal abuse and insult.
Narcissists are impulsive. They undo themselves by behavior that seems oddly stupid for people as intelligent as they are. Somehow, they don't consider the probable consequences of their actions. It's not clear to me whether they just expect to get away with doing anything they feel like at the moment or whether this impulsiveness is essentially a cognitive shortcoming deriving from the static psychic state with its distorted perception of time.
Narcissists hate to live alone. Their inner resources are skimpy, static, and sterile, nothing interesting or attractive going on in their hearts and minds, so they don't want to be stuck with themselves. All they have inside is the image of perfection that, being mere mortals like the rest of us, they will inevitably fall short of attaining".