In little over a month, Pippa Middleton will walk down the aisle at St Mark's church in Englefield to marry James Matthews.
Ever since the couple's engagement was announced last summer, speculation has been rife about who will design the wedding dress, with British labels including Jenny Packham, Ralph and Russo and Suzanne Neville all in the running.
But among the guests at the nuptials will be another attendee whose choice of outfit for the day will be closely watched: The Duchess of Cambridge.
The bride's big sister has become a pro at dressing for a tricky roster of occasions, from royal church services to foreign tours and charity galas. But pitching her look perfectly for her sister's wedding is set to be one of her most fascinating sartorial challenges yet.
Pippa famously almost stole the show at the 2011 wedding of Kate and Prince William with her figure-hugging Alexander McQueen maid of honour dress but it's unlikely that Kate will now want to reciprocate that move.
According to the Daily Telegraph she will only give a reading at the service, rather than perform a role in the wedding party, although it has been confirmed that Prince George and Princess Charlotte will be page boy.
The Duchess will attract attention no matter what look she chooses, but here are some of the style strategies she might adopt ...
A recycled look?
This is a modus operandi which Kate has employed at numerous other weddings in the past, opting to wear a look she has been seen in before so as not to overshadow the bride.
At the wedding of Zara Phillips and Mike Tindall, she wore a pale yellow coat by Day Birger et Mikkelsen which she had first debuted in 2006 for the wedding of Laura Parker-Bowles.
Similarly in May 2013, she attended the nuptials of William Van Cutsem and Rose Ruck Keene wearing a black and white spotted dress from Topshop which she had also worn for a visit to the Harry Potter experience just weeks earlier.
So which looks might the Duchess bring out of her wardrobe for Pippa's wedding?
She has a plethora of previous outfits to choose from but that selection is seriously narrowed down when the requirements of wedding guest dressing and the late Spring timing are taken into consideration: she will likely avoid wearing white but pastel hues and a hat are almost dead certs.
This delicately patterned, pale grey Erdem dress which Kate has worn for a previous wedding as well as Trooping the Colour in 2012 is a sophisticated option.
The bubblegum pink, pleated Emilia Wickstead dress which Kate has also been seen in twice previously is also a pretty but pared-back possibility.
On a visit to the Netherlands in October, Kate wore this lilac skirt suit by Catherine Walker and co. for the first time, drawing comparisons with Jackie Kennedy's neat-chic style.
It's quite a formal look but for a chillier day, it could make for a polished choice which says "don't look at me, look at my sister".
A high street choice?
Another way to dress subtly yet chicly for Pippa's wedding could be for the Duchess to choose an affordable look.
Kate is known for her canniness in mixing high street pieces with her more high-octane looks and her sister often does the same. In fact, the evening before her engagement was announced, Pippa was photographed wearing an L.K Bennett dress.
L.K Bennett would be a natural choice if Kate was in search of an elegant but unshowy guest look, renowned as the label is for its vast array of lovely dresses.
This cornflower printed dress debuted last summer exemplifies its elegantly dressed-up look and there are plenty of pieces in store now which would be great choices, from this pale pink, petal lace midi dress to this delightful embroidered floral style.
At Hobb's- the label which Kate wore for her last visit to St. Mark's church at Christmas- there is also an excellent selection of wedding-worthy new season dresses including a yellow lace shift and a butterfly-printed, cornflower blue design.
The Duchess has also favoured Whistles, Topshop, Zara and John Lewis over the years, all of which have plenty of brilliant options.
Something special by one of her favourite designers?
The Duchess of Cambridge has amassed a circle of favoured designers to whom she often turns for bespoke pieces for special occasions- think Alexander McQueen, Jenny Packham, Catherine Walker, Alice Temperley and Emilia Wickstead.
Although Pippa's wedding is a private family occassion, that's not to say that Kate might not want something special for the day.
A chance to try something new?
Marchesa, Preen and See by Chloe are all labels which Kate has introduced into her repertoire over the past few months, a suggestion that she is feeling bolder than ever when it comes to experimenting with her style.
Pippa's wedding could be the perfect setting for Kate to try a look by another unfamiliar designer.
Attico's beautiful silk floral tea dresses have the ideal bucolic feel. Gül Hürgel's flattering retro designs are ladylike, crisp and timeless. Or London-based designer Rejina Ryo does modern but elegant dresses with a twist.
Now that Royal Ascot have given jumpsuits the go-ahead for admittance to the Royal Enclosure, they will surely be everywhere this summer season.
Which is why Kate should take her chance to show she's au courant and pick a jumpsuit for her sister's ceremony. Alice Temperley's blush pink sequinned style or Diane Von Furstenberg's floral kimono style would both be excellent choices.