There’s an undeniable aura of glamour to any discussion of the Mitford sisters; each as effervescent - and acid - as a glass of champagne, their upper class eccentricities and wildly divergent life paths make them a rich vein for fans of juicy historic drama. It’s not unfair to say that in many respects they were the English prewar equivalent of the Kardashians, up to and including marriages to self-proclaimed fascists. Amy Rosenthal’s The Party Girls at the Oxford Playhouse beautifully plays on this reputation for privilege and frivolity, as the double meaning of its title implies; there’s a lot of bad politics you can hide under the guise of just larking about.

Glam though the Mitfords certainly are, The Party Girls is about exposing the rot beneath the silks. Told in non-chronological order and in large part through the eyes of communist bête noire Jessica ‘Decca’ Mitford (Emma Noakes), we watch as fascism creeps like a festering wound through the family line, squared against Decca’s later attempts to fight that corruption both in herself and the world at large. Noakes is the play’s emotional heart and she is beating a mile a minute, the civil war between her ideals and her upbringing playing out with perfect clarity in her will-they-won’t they romance with Joe Cohen’s Bob Treuhaft. And Simon Kenny’s shifting platform set design lends the impression that Decca’s past at Swinbrook House is constantly in pursuit, or sometimes lurking right behind her.

Of course, if you’ve come looking for arched eyebrows and barbed words, you certainly won’t be disappointed - all the sisters get their share of digs in, but the best come from (who else?), novelist Nancy, played with glorious grand-dame aplomb by Kirsty Besterman. Even convalescing in her Paris residence in 1969 she has the good decency to do it in a silk robe while reading poor Decca to filth, comparing her to a dowdy housewife in a cornflake commercial. Rosenthal’s comedic writing is wickedly pointed and a much needed tension breaker amidst the play’s more grim subject matter.

Because in the moments where the Mitford’s Nazism come to the fore, Rosenthal uses the same idiosyncrasies we’ve found charming in them before to illustrate how easily fascism can be laundered into everyday life under the veil of politeness, decorum and ‘good British values’ - the gentility of evil, if you will. People are keen to forget how sympathetic the British aristocracy was to the aims of the Nazi party prewar. Diana Mitford’s rhapsodising about what a ‘hon’ her ‘Kit’ is gets a lot less cute when you know she’s talking about Oswald Mosley, and her final interaction with Decca, a chillingly curt denial of the Holocaust (taken almost verbatim from Diana’s 1989 Desert Island Discs interview), is delivered in a Dior suit lounging on a Parisian chaise, followed shortly by “you did look smashing, darling”. Even the grim portents of wartime in the papers are washed down with socialites’ endorsements of the new vitamin A Pond’s cold cream, a grim 1930s precursor to Instagram feeds alternating between influencers and massacres.

Similarly, Ell Potter’s Unity’s schoolgirlish obsession with Hitler takes on an almost Annie Wilkes-ish turn at points. In a stellar cast, this was one of my favourite performances of the night, as moon eyed crush gives way to dogged conviction and a mania that crosses the line into physical violence. It might seem like a sitcom style set contrivance, but Unity and Decca really did draw a chalk line down the centre of their room so one wall could be plastered in swastikas and the other with Bolshevik posters, an insulated, teenage trivialisation of one of the greatest moral battles in living memory. And truly what better example of the gentility of evil than to watch Unity Mitford agonise over whether Eva Braun is prettier than her?

At a point when we are backsliding to the far right like someone cut the brakes on the world, while at the same time constantly being told to be polite and civil to those perpetrating hatred, it was genuinely refreshing to see Decca’s arc resolve in a way that does not lead to her trying to find some sort of happy medium with her Nazi sympathiser siblings. In her final encounter with Diana the clipped RP accent drops and she yells with her whole chest - where the others forever implore “do forgive”, she orders “do admit”. Rosenthal knows that to offer a fascist a middle ground is to give them the ground entire, and no happy ending lies there. Where it does lie is in the arms of a nice Jewish human rights lawyer who sees all you’ve been as well as all you are, and loves both wholeheartedly.

It's Sisters Week at the Oxford Playhouse. While the Brontës are dying of consumption in the Burton Taylor Studio at the back of the building, the Mitfords are going full Adolf in the main house.

The British have an abiding fascination with the Mitford Sisters. They’re like monsters in debutante form, Frightening Young Things. Buried barely 18 miles from the Playhouse Box Office in the stunning, chocolate-box village of Swinbrook, pretty graves all in a row, they are proof that some corner of a British field is forever fascist.

We just can’t get enough of them. There are at least thirty books that tell their story, from the Collected Letters to Hitler’s Valkyrie. As recently as this summer, TV audiences have been treated to Outrageous, which explores the lives and loves of this bizarre family. But Amy Rosenthal’s new play wisely avoids tackling the whole saga, and focuses on the heroic Jessica (Emma Noakes in a performance oozing dignity and wit), who revolted against the family tradition of right-wing extremism and became a campaigning socialist – even marrying a Jew, which can’t have gone down well with the in-laws.

The play is composed of selected scenes from the lives of the sisters, ranging from 1932 to 1969. Again like Jane Eyre next door, it jumps backwards and forwards in time, comparing outcome with intention, and youthful potential with dying pain.

Rosenthal’s writing echoes across the decades with lost ambition, regret and bitter idealism. Each of the sisters fights her own corner with the vibrancy of total self-belief and centuries of privilege. And somehow, Rosenthal even imbues the fanatical extremist Unity (truly more of a Hitler Girl than an It Girl) with moments of poignancy. At one stage she sits in the window yearning for the Führer while tugging forlornly at a plucked flower. When rebuked by Nancy for damaging it, she says, ‘It was only the leaves, not the petals’, and the hopelessness of her delusion feels tragic.

At the climax of the play, in 1969, an aging Jessica confronts Diana (who married Oswald Moseley and actively supported the British Fascist Movement) and begs her to admit that she was wrong. Diana gives not an inch. Instead she pointedly looks to the future and tells Jessica that antisemitism will have its day. (For a truly chilling listen, I recommend searching for the Diana Mitford Desert Island Discs on BBC Sounds. Sue Lawley also probes her fascist history, and the elderly Duchess is unrepentant to the end.) Prejudice is inherited, and the takeaway from this play is about the effort needed to escape family influence.

Despite the lyrical writing and a group of impeccable performances, The Party Girls does drag in places. 90% of this play is stasis filled with discussion. Rosenthal is aiming at a Three Sisters vibe with a Mitford skin. But she is no Chekhov. His ability to convey exquisite pain masked by everyday pleasantries would be ideal for the Mitfords, but the chit-chat here frequently conceals rather than reveals the inner lives of the characters. Consequently there are longueurs where the party girls are more dreary than sparkling. And while their voices and language are brilliantly observed and convincing, the debate they have feels like what it is: a debate. It’s on the surface, like an episode of Question Time, rather than submerged and resonant. Consequently, when Unity (spoiler alert but it’s a well-known fact) pulls out a gun and shoots herself in Munich, it comes out of nowhere.

The play looks gorgeous, with a wafting, diaphanous gauze of dark blue satin whisking aside for each scene, as if offering us a glimpse of one of those stunning 1930s ball gowns. And with a minimum of fuss, Simon Kenny’s sparse design conjures the kitchen of Asthall Manor, a back alley in New York, the English Garden of Munich, and a posh apartment in Versailles, while lightning costume changes fling the sisters from youth to age and back again in seconds.

If The Party Girls is a collection of scenes from the lives of interesting people, it doesn’t always pick the most interesting scenes. But it has a true purpose, it is full of heart, and it directly addresses one of the key issues of our day: how do you talk to family members with whom you profoundly disagree?