In just over a week, Meghan Markle will walk down the aisle with Prince Harry in what is surely the most anticipated TV event of the year. The American actress will have a new title, a new husband and, most notably, a new family of in-laws to ingratiate herself with.
To peddle the old cliche; you don't just marry the one, you marry the entire family. On May 19, Meghan will become a card-carrying member of 'The Firm', but it's the relationship with her future sister-in-law, the Duchess of Cambridge, that we'll be watching most intently.
They say a loyal sister is worth a thousand friends. A sister-in-law - loyal, royal or otherwise - is something else entirely.
Prince Harry assured us that "Catherine has been absolutely amazing" in helping Meghan prepare for public life during his first TV interview with his future wife, but there's still little evidence to suggest these women have established a genuine bond.
As former 'commoners' with no blue blood, Meghan and Kate Middleton should have plenty to talk about.
Kate married into the Royal Family seven years ago and, by now, she knows the territory. She has made sense of the language and traditions, the peculiarities and eccentricities.
This type of sister-in-law can prove to be a powerful ally to an outsider. She can tell you not to take it personally when Aunt Betty passes a withering remark. She can give you the inside edge on that feud that nobody talks about. She can confirm, from an objective standpoint, that this family are indeed dysfunctional beyond belief.
Kate has the map for anyone new to these parts - the question is whether she's willing to give Meghan directions.
Sister-in-laws that marry into a family often bond through a common enemy - usually a particularly meddling in-law. But they can become hostile towards one another too, especially when they are as close in age as Kate and Meghan, who are just a few months apart.
Kate will always have a higher royal status than Meghan, but the public will ultimately decide on their 'People's Princess'.
The women may also compete for favouritism within the family. Meghan has already won over the corgis who apparently lay at her feet during her first meeting with the Queen.
Sarah Ferguson, aka Fergie, established a similar affinity with the Queen when she married into the Windsors in 1986.
She was a horse-lover like her mother-in-law and the former wife of Prince Andrew often accompanied the Queen on private rides through the grounds of Windsor Castle.
Like Meghan, Fergie arrived on the scene a few years after Diana and Charles married. She had a lower royal status than Diana but this didn't make the Princess of Wales feel any less insecure.
"I got terribly jealous of Fergie," she admitted in The Diana Tapes. "And she got terribly jealous of me."
The late princess also talked about the first time Prince Andrew invited Fergie to Highgrove.
"Suddenly, everybody said, 'Oh, isn't Fergie marvellous, a breath of fresh air - thank God she's more fun than Diana'. I felt terribly insecure. I thought maybe I ought to be like Fergie. And my husband said, 'I wish you would be like Fergie - all jolly. Why are you always so miserable?'"
Fergie and Diana went on to become close friends and fellow mischief-makers but the fear of comparison was always there. Diana was deep and contemplative; Fergie was fun and gung-ho.
Kate and Meghan have been compared in similar terms. Meghan is expected to modernise the monarchy with her touchy-feely brand of Californian liberalism, which makes Kate look staid in comparison.
Motherhood can become another point of difference. Kate has given birth to three heirs to the throne and royal-watchers say she may emulate the Queen by having a fourth. Meghan, on the other hand, may choose to have a smaller family.
Jackie Onassis' sister-in-law Ethel gave birth to 11 children, which led to Jackie christening her "a baby-making machine - wind her up and she becomes pregnant". Ethel, for her own part, is said to have nicknamed Jackie 'The Debutante' - a jibe at her demure temperament.
It's hard not to liken it to Tennessee Williams' Cat On A Hot Tin Roof, in which childless Maggie felt she was being pitted against her sister-in-law, the "monster of fertility" and mother-of-six Mae.
Kate and Meghan are lucky that William and Harry don't have sisters of their own. Received wisdom tells us that the mother is the trickiest in-law to deal with, but ask around and you'll discover that sisters can be just as meddlesome.
Some sisters can be very protective of their brothers - their younger ones especially - and this can sometimes manifest as intimidating, controlling and downright rude behaviour.
Others can go out of their way to build a sisterly bond, which can come with its own challenges when they start inviting you to bottomless brunches with 'the gals' and texting you inspirational quotes about the power of love.
The sister of the bride can be a troublesome presence too. When there is a close relationship, as with Kate and her sister, Pippa, this type of sister-in-law can quickly become the third wheel.
Pippa - or rather Pippa's posterior - stole the spotlight on Kate's big day. The Middletons probably didn't mind the scene-stealing but the Royal family may have had different opinions. Pippa has since walked down the aisle herself and she is said to be expecting her first child. She used to be a constant presence in her sister's life, but family obligations may soon take her in a different direction.
Kate could benefit from having a sisterly figure around. Meghan could too, given that her half-sister Samantha is busy writing The Diary of Princess Pushy's Sister, a tell-all book about their relationship. These two women will be constantly pitted against one another but we ought to remember that sisters-in-law can also establish meaningful relationships. They can be dear friends, loving aunts and, like Diana and Fergie, they can become as thick as thieves too.
As long as Meghan and Kate can negotiate the dominance hierarchy and not jostle for favouritism, they could forge an authentic friendship. After all, it would help to have a friend on the inside.