Six aristocratic sisters, each of whom come of age between the wars and grows up to become: a novelist; a dairy farmer; a fascist; a Nazi; a Communist; a duchess. Therein lies the root of my fascination with the Mitford sisters: Nancy, Pamela, Diana, Unity, Jessica and Deborah. (There was also a brother, Tom, understandably overshadowed by his sisters and tragically killed in action at the end of the second world war.)

What made the young girls in the nursery of Asthall Manor grow up to become these fascinating women? We know a lot from their combined prodigious literary output. Not just the novels of Nancy, or the memoirs of Diana and Jessica, or even the chicken-raising tips of Deborah but their letters. The six sisters wrote to each other frequently over their long lifetimes, even throughout fractious disagreements and political standoffs. There are also several other published volumes of their letters to other great personalities and writers of their era.

They wrote, I think, somewhat self-consciously, always aware of their celebrity – their mother once said, “Whenever I see in a headline ‘peer’s daughter’, I know it’s going to be about one of you children” – yet they are deliciously unguarded, gossipy, spiky and witty. There are several exhortations, obviously ignored, to “burn this”. Their fascination is nonetheless a conundrum.

Deborah, the Duchess of Devonshire, was probably the nicest of the lot, and Pamela the most stalwart, but they were on the whole a fairly unsympathetic bunch. Nancy was the unhappiest, largely repressed in her letters, instead using her bitterness to produce the most dangerous rapier wit against those she disliked. Yet I’d give anything to drink martinis with the woman who said: “I am sometimes bored by people but never with life.”

There was sibling rivalry aplenty, a constant thrum of bickering, and there are frequent discrepancies as to what happened in their childhood and what their parents were really like, which is reassuring to anyone with tricky family dynamics of their own (er, everyone?). Which makes it all the more remarkable that Jessica, the committed Communist (she and her first husband ran away to Spain to fight Franco), would secretly meet with Unity, who had gone to Munich in order to befriend Hitler (and succeeded). They understood that although one’s political beliefs are of course fundamental to one’s values, we are all made up of complex layers and it is possible to disagree with the people one loves; a nuance of discourse that we are in danger of losing today.

With their unlikeable politics and an arrogance of their class that propelled them through life, we shouldn’t like them but we do. They were absolutely themselves, caring little what others said, and loyal to each other and their causes, even in conflict. They represent so much of that intense period of the 1930s that is in parallel with today – understanding them may help us get to grips with our own troubled 21st century.

The Mitford Trial by Jessica Fellowes is published by Sphere on November 5, £13.99. Inspired by a real life murder in 1933, at its heart are the six Mitford sisters.