The five golden rules of good sister-in-law behaviour

Kate Middleton, Duchess of Cambridge and Meghan Markle, Duchess of Sussex. Image: Paul Marriott/Alamy Live News
Kate Middleton, Duchess of Cambridge and Meghan Markle, Duchess of Sussex. Image: Paul Marriott/Alamy Live News Photo: Alamy

There are few things Kate Middleton and I have in common. Certainly, when it comes to her tiny waistline and stallion-esque pins, we're oceans apart.

But there's one experience I can claim to share with the Duchess of Cambridge. And that's the advent of a glossy, more sophisticated interloper dazzling her way into an established family group.

I speak, of course, of sisters-in-law. In my case Fiona, my brother's blonde, slim, whip-smart, effortlessly stylish wife. When she first married into our family, there could have been huge scope for the type of brittle interaction that has been rumoured to be playing out between the Duchesses of Cambridge and Sussex.

It is something that I don't doubt many sisters-in-laws will be able to relate to. "The expectation to become close to someone simply because you've married into the same family can cause strain," says Louise Tyler, relationship counsellor and founder of the Personal Resilience clinic in Cheshire. "You're expected to spend time together and get along, yet there may well be differences in values, parenting styles or even broader attitudes to family and money."

Of course, if the late Diana, Princess of Wales were still here, she would probably have knocked the duchesses' glossy heads together - not least because she'd understand the potential impact that a Kate/Meghan fallout would have on her beloved sons.

Diana could have offered sober council over the sister-in-law dynamic, especially since her own relationship with Sarah Ferguson hurtled from BFF to persona non grata in less than a decade. But does the sister-in-law relationship really have to be so challenging?

Having been one for more than 30 years (I was a mere teen when my older brother married), and now in possession of a few more, I've learnt that while there's plenty of ammunition for the relationship to go nuclear, it can also be utterly wonderful.

A brilliant sister-in-law can be your ally, there to watch your back and, unlike all your other friends, the only person who really gets the nuanced bits about the family you've both married into.

Strangely, there's little research about the sister-in-law dynamic, with joke book writers and university researchers preferring to analyse that other familial relationship - the mother-in-law, instead.

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Surely anthropologists could have a field day studying this relationship with a woman who you haven't chosen as your friend. But that's rule number one for good relations. Your sister-in-law doesn't have to be your best pal. Trying to force a relationship will just cause it to implode. Much better to let it evolve naturally.

My oldest sister-in-law is a fiery Glaswegian with some bonkers ideas. But she's also super-clever, with a heart the size of Texas, and will do anything for our family (especially my kids).

In fairness, perhaps one of the reasons I get on so well with my older sister-in-law is the fact that she is six years older and lives abroad, not across a Kensington courtyard. It means the relationship can breathe.

The problem for Meghan and Kate is that they are boxed in like two finely bred show ponies. Badged as global icons, it's a similarity that can only create competition. Which is why they would do well to observe the second rule of good sister-in-law behaviour: never try to compete.

If you're new to the sister-in-law game, the third rule to know is learnt from those with shop-floor experience. Kate, after all, is a road map to joining The Firm, having never put a well-shod foot wrong since being recruited.

Rather than doing things her way - as the fiercely opinionated Meghan is alleged to do - the newbie would be wise to soak up that knowledge.

Another vital rule for sisters-in-law is to keep any problems between yourselves. Don't involve your spouses, otherwise you'll find it hard to be forgiven for causing a family rift. This is especially true in the case of Princes William and Harry, who bonded so profoundly over the untimely loss of their mother.

But, ultimately, the best way to seal the deal with a sister-in-law, no matter how different you both are, is to be nice to each other's children. When Meghan gives birth next spring, this will be the real opportunity for the duchesses to find that authentic connection.

I don't doubt Meghan and Kate will resolve their differences and see the benefits, rather than the drawbacks, of forging a relationship. There's little to lose and so much to gain. (Though Fiona, if you're reading this, couldn't you look less gorgeous just once in a while?)

The Daily Telegraph, London