Two sisters, twenty months apart, the same childhood in Berkshire, the same mother teaching them the same things. And then one day - 29 April 2011 - one walked into Westminster Abbey as the bride of a future king, and the other walked in three steps behind her, holding the train.

From that day the wardrobes of Kate and Pippa Middleton have headed in opposite directions, and it is not a question of taste. It is a question of what each is allowed.

Kate dresses like an institution. Structured dresses, midi lengths, single-colour suits, carefully chosen neutrals and deep tones. Alexander McQueen, Catherine Walker, Jenny Packham, Emilia Wickstead - names that recur with a predictability that is not accidental. Every fashion decision she makes is also a protocol decision. It must not be too short, too bright, too new, too cheap or too expensive. The space between those five prohibitions is narrow, and she moves through it with a precision most would call elegance but which is in fact discipline.

Pippa, who turned 43 on 6 September, dresses like someone with nobody to answer to. Floral prints, light fabrics, pieces that look picked up in the countryside, sportswear that is actually worn. She reaches faster for a strong colour and a shorter cut. Not because she has better taste - because her dress is not analysed in columns about constitutional questions.

The difference is neither accidental nor new. Of her dress at her sister's wedding, Pippa said: "The idea was not for it to be a significant dress, but for it to fit in with her train." That is the sentence of someone who already knew where she stood in the frame. In 2013, before she met James Matthews, she also said: "If I were to get married, I would do it in my tennis shorts." Of her own childhood she said she was "an unfeminine and very determined girl".

Matthews, whom she married in 2017, is a businessman, a former racing driver and the Scottish laird of Glen Affric. With that, Pippa entered a world that has its own rules, but nobody broadcasts them live.

Of the relationship between them she says it has not changed: "We are very close, we support each other and ask each other for opinions." That is a statement from 2014, given to American television, and since then neither of them has said anything that contradicts it.

What changes between them is not the taste but the weight of the consequence. For one, the dress is a choice. For the other, it is a statement the whole country reads.