.
As for the whole Fergie tiara deal....I think that the Queen has always had her favorites. Why else has Sophie got a tiara made up of historic pieces, a royal order and some other jewels were it not for being the Queen's favorite. I believe how the Queen loans out jewels says how she feels about people. Then again the Queen is not as generous loaning out her pieces when you compare her to other royal houses. So this could be all we see from Kate, in terms of jewels, for a long time. Then again William could always buy her something. He after all doesn't seem that generous. The ring came from his mother and the gold of her band came from the Queen so he needs to step up.
I am quite sure that you are right about [non!] receipt of the Royal Family Order as being indicative of how the Queen feels about [female] family members: the only two members of the Royal Family who did not receive it are /were Princess Michael of Kent and Sarah [when eligible - i.e. during her marriage] [Catherine has not yet been so-honoured, either, but that is surely because she has only recently joined the BRF!] It is actually quite a snub to PMK, because although she undertakes very ,very few 'official royal engagements', she is nevertheless treated as part of the 'wider BRF' with invitations to State Banquets etc. Moreover, her own lady-in-waiting Lady Kitchener-Fellowes was honoured by the Queen [with an LVO if my memory serves me correctly, which it usually doesn't!!]. It must be quite a come-down to have your Lady in Waiting honoured by the Queen without having received any 'recognition' yourself!
With regard to the tiara issue etc that you mention vis-a-vis Sarah, I don't personally think that this was
initially an indication of how HM viewed Sarah. Don't ask me for a source, as I am aware of this only 'from my mother's knee', but I understand that when any lady marries into the BRF, the Queen carries out her own 'informal assessment' of the lady's 'own jewellery stock' to see what she 'needs' in order to 'function' as a Royal. Basically, the Queen sees that the new Royal lady in question will have access to a tiara, [for state occasions etc] and suitably matching jewellery etc. Most Royal Brides of the 'previous generation' [ Duchess of Kent, Duchess of Gloucester, even PMK] all married family members with access to tiaras etc. Any deficiences are then 'made good by the Queen'
With regard to the newer generation of Royal Brides, there was never going to be a problem with Diana - for a start, not only had she access to the Spencer tiara but as the wife to the future heir of the throne, she was never going to be short of jewels [and this also applies to Camilla of course].
In Sarah's case, following her engagement to Prince Andrew, she immediately went to Garrard [the then-Crown Jeweller] and arranged to
hire a tiara for the wedding. When the Queen heard about this loan, she immediately
bought the tiara for Sarah and generously supplemented this with a necklace, bracelet and earrings etc to complement this [this was the set of jewellery part of which you may remember was then temporarily stolen from Sarah at JFK airport a few years ago]. I am quite sure that had Sarah not beaten a path to Garrard so quickly, the Queen would [at that stage of Sarah's Royal Career, when of course the newly-engaged Sarah was at the height of her popularity] have stepped in and 'permamantly' loaned her a tiara from the Royal Collection. Prince Andrew also bought his wife a significant amount of 'private' jewellery from Cartier and the like [at the time of meeting Andrew at Royal Ascot in 1985, from memory Sarah had only a Cartier Santos Watch [apparently a present from Paddy McNally] and a necklace with GB on it round her neck [from Major Ron]. Indeed, because so many of the pieces of jewellery which Diana was given by the Queen came from HM's own 'private jewel pool' with the implication that these were 'gifts-for-life' rather than 'outright ownership] the irony was that Sarah in theory did 'much better' because the pieces she was given were 'hers, not hers for life].
And fortunately for the Royal Jewellery collection, as Sarah's marriage in the event did end in divorce, the tiara issue turned out to be a 'happy accident', as Sarah did not acquire a 'Royal Piece'. Incidentally, off-topic but I will naughtily mention it here - when Sarah thinks she is poor, she forgets that her private jewellery collection is easily worth well over a £1m, on its 'contents' alone, without taking into account the increase of value because of its provenance [i.e. 'owned by the former Duchess of York']
Sophie did not of course 'rush to Garrard like Sarah had done' and thus other provision had to be made for her. I think it is fair to say though that in any event, Sophie's tiara was always 'surrounded' by a bit of mild controversy - I am no royal jewellery expert [unlike some of the clever people here] but I am aware that it was apparently 'put together from other royal pieces' and so, perish the thought, had Sophie's marriage foundered and a fight arisen over her jewels, the BRF would not have 'lost' very much from their heritage collection even if Sophie had 'retained the tiara'.
Hope some of this helps,
Alex