Dear Richard Madeley: My narcissist sister is going after my friend’s husband – should I warn them?

She used to be a model, and I know how irresistible she can be when she is focussed on a man

My sister is a narcissist but, strangely, quite likeable. She used to be a model and, at 40, she can still silence a room when she walks into it. She’s had lovers galore but never met the man she wanted to settle down with, until a few weeks ago. Unfortunately, that man is my best friend’s husband.
This friend and I have known each other since school and we were inseparable throughout our childhood. I was her maid of honour and she was mine. Our husbands get along and we often go on holidays together with our children. Both our husbands are fabulous looking, we’re both comfortably off – we have a great deal to be grateful for. 
I was always hesitant about including my sister in any social plans with the four of us because I didn’t want her to make herself the centre of attention. Unfortunately, she dropped over when we were having dinner and her attention was laser-guided towards my friend’s husband. She told me after they left that she intends to pursue him.
I don’t know what to do. My loyalty is to my sister, but she has never been a friend to me. I know how irresistible she can be when she is focused on a man. She is relentless. 
Should I warn my friend (or her husband) about my sister’s intentions? Or let nature take its course? Either way, I worry I’ll be letting someone down.
— Audrey, via telegraph.co.uk
I have a friend who knows a good deal about narcissistic behaviour. He’s made something of a study of it, so I consulted him (and did some additional research) before replying to your letter.   
True narcissists have no real interest in the feelings, concerns or behaviour of others, unless they are directly and personally impacted by it. They see themselves as the centre of the universe: everything revolves around them and their needs, desires and plans. It’s not that they consciously choose to be profoundly selfish: the egotistical behaviour is simply embedded in their unconscious.
So I’m sorry to say that appealing to her better nature is likely to be a waste of time because, in this context, she doesn’t really have one. She’s simply seen something, or someone, that she wants. Ergo, she feels entitled to have it. If you confront her, she will probably, genuinely, not understand what you’re talking about. The fact that she so casually shared her intentions towards this man with you reveals much about her mindset. It simply didn’t occur to her that you might disapprove, or be alarmed on your best friend’s behalf.
So yes – you should talk to your friend, and possibly her husband, too. Warn them. Let them know what kind of person your sister can be and what she is capable of, however charming and attractive she may seem on the surface. If she seduces this man, and quite possibly destroys his marriage in the process, there’s every likelihood that she’ll toss him aside the moment she tires of him. Damage done; wreckage casually left behind.
Empowering your friends to protect their marriage is not an act of disloyalty towards your sister. You’re simply handing them important information. What they choose to do with it is up to them. But forewarned is forearmed.
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