I would just like to ask why Beatrice and Eugenie aren't like other princesses?and what I mean is that why don't they have at least their set of royal jewels and if they have a tiara like other princesses insted they wear big or funny hats and suits. But above all why they as princesses of York don't wear a sash or tiara ?
Please can I try to help with this one? so far as Tiaras are concerned, it is british formal etiquette [I can't speak for other jurisdictions] that ONLY married women can wear tiaras. This comes from the same sort of custom-and-practice that provides that you say 'How do you do?' when you are introduced to someone [but not of course presented to the Queen.]. People here like to know the sources of my views, so all I can say on this one about tiaras is that this is the sort of information I just absorbed automatically from my family as a child. Any good etiquette book should be able to confirm about the tiara point.
By custom and practice, a bride who arrives at church is permitted to wear a tiara, although at the time of her arrival, she is of course an unmarried woman.
Don't confuse a tiara with a Coronet; if you can find pictures of the present queen's coronation, you will probably notice that the infant Prince Charles and Princess Anne (as she then was) were wearing 'something' on their heads - these were Coronets [i.e. the infant Princess Anne was not wearing a tiara]
I would also point out that you don't have to be Royal or from a noble background [I'm speaking about UK custom only, I can't speak for continental and other jurisdictions] to have a tiara. Indeed, a good few non-noble families own tiaras and many aristocratic families do not, having sold their tiaras [money problems possibly, e.g. to meet inheritance tax on death] and indeed from memory, one of Princess Margaret's Tiaras was auctioned by Viscount Linley. I am sure one of you royal-watchers will be able to confirm this.
It used to be a strict rule of etiquette that Tiaras were only worn with what is called 'White Tie' - i.e. at the most formal of occasions when gentlemen are required to wear tails and white tie, not 'so-called 'black tie' [which basically means Dinner Jacket, black bow tie etc etc. [Tiaras are also worn by Peeresses at the State Opening of Parliament].
Unkind people used to snipe at Princess Michael of Kent when she issued a photograph of herself in an exquisite gown, wearing a tiara; Prince Michael was photographed with her wearing 'black tie', not white tie.
Princess Michael obviously had the gift of special insight however; a few years after the photo was taken [I expect it is on the web somewhere, it is certainly in various books], the Queen held her special party in 1990 to celebrate the Queen Mother's 90th birthday, Princess Margaret's 60th Birthday, the Princess Royal's 40th birthday and Prince Andrew's 30th birthday. It was specified that 'White tie' was not to be worn, but that tiaras could be; the press were briefed that this is because the queen, knowing how ladies with tiaras loved to wear them and that there were few formal occasions any more when they could do so, specified that Tiaras could in fact be worn.
A bit of 'gossip' now; people who reckon they know about these things always say that there are only two hairdressers in London who know how to put on a tiara properly: Michael of Michaeljohn and Hugh in Ebury Street. There is a special knack to dressing hair for a tiara - the trick is to make sure that the velvet band on a tiara (i.e. base of tiara) is NEVER visible -- your hairdresser should be sufficiently competent to dress one's hair so the velvet band is never seen [don't foget that a tiara is mounted on a velvet band - a tiara is not fully round, like a crown or coronet would be]. Also, it is best if a hair piece is not used to try to 'bulk out' hair - fortunately Catherine has lovely hair - I remember though how you could clearly see the hairpiece used to 'pad out' the hair of Viscountess Linley at her wedding.
Another bit of information; during days when there is a 'White Tie' event on [usually in the evening, but the State Opening of Parliament is during the daytime], there may be as much as several hundred ladies needing their hairdresser to fix their tiara...and you can't necessarily get the appointment you want right just before the event. You therefore have to go and get your tiara fixed on when you can and so it is quite common to see ladies walking around London at 11am in Jeans, with scarves attempting to conceal the newly-fixed tiara beneath.
Hope this helps,
Alex