Kenneth MacMillan created Las Hermanas ('The Sisters'), for Stuttgart Ballet. Like Lorca's play The House of Bernarda Alba on which it is based, Las Hermanas is a tense psychological drama about sensuality under harsh repression and the emotional and violent consequences that follow. It tells the story of five sisters, all unmarried, who have been cowed into conformity by their tyrannical mother. The eldest is engaged, but the family is torn apart when her fiancé is seduced by her youngest sister. A third sister betrays them. The fiancé is banished, the elder sister is condemned to a lonely spinsterhood and the youngest hangs herself in shame.

MacMillan had to persuade audiences that dance could be as apt a medium for the story as the spoken word. In the event, his choreography stretched ballet to new limits of dramatic expressiveness. “It is classical, certainly, in its language but transmitting classicism into ferocious drama.” wrote James Kennedy in The Guardian. According to Monica Mason, "Kenneth would have been hugely attracted by the claustrophobia of this house and these young repressed women; it was right up his street."

It was the designer, Nicholas Georgiadis, who suggested to MacMillan the balletic possibilities of Lorca’s original. Georgiadis’s highly realistic set, the distillation of Lorca’s oppressive atmospheres, is like a cloistered prison and follows the playwright’s instructions exactly. “A very white room in Bernarda Alba’s house. The walls are white. There are arched doorways with jute curtains....” Frank Martin's score is similarly apt, the sound world described by the solo harpsichord eerily claustrophobic.

MacMillan had an outstanding first cast in Stuttgart that included Marcia Haydée, Birgit Keil and Ray Barra. At the outset, the principal characters are strongly established; the elder sister’s introversion with clenched fists, and a contrapuntal use of upper and lower body; the jealous sister with waspish meanness, quick turns, and accusing arms. Once the younger sister has tried on her sister’s wedding veil, an atmosphere has been created, the scene set for what is to come. The groom has an animal insolence but in the pas de deux with the bride-to-be, she is spinsterish, all rigid arms and tight fists. She shrivels when she realises they are being watched. Then the groom dances with the younger sister – this time the dance is abandonedly sexual. The jealous sister tells the household. The denouement follows and the mother banishes the groom; the sisters contemplate a bleak finality. The youngest sister is discovered hanging as the curtain falls.

Reviewing the Stuttgart production at the Edinburgh Festival later that year, The Times reviewer (un-named but almost certainly Clive Barnes) wrote that “MacMillan's use of literature tends towards the cavalier and this ballet is no exception, but he has extracted from the Lorca play an imaginative libretto of considerable dramatic impact.” Alexander Bland, reviewing the 1971 Sadler’s Wells production for The Observer, noted that Lorca’s original had been “translated into dance terms with a sustained intensity”. But James Kennedy, reviewing Western Theatre Ballet’s 1966 production in The Guardian cavilled at the depiction of the younger sister’s suicide; “a shock all right, but not the right sort of shock in the context.”

In 1966 Peter Wright directed a BBC2 studio version of Las Hermanas. “I brought over Marcia Haydée and Ray Barra, along with some dancers from the Royal Ballet, including Georgina Parkinson and Monica Mason. While keeping to the actual set designed by Georgiadis, I was able to enlarge it and make it much more as if it had been created for television rather than for the theatre. One could get in close to the action. It worked extremely well.”

In 1966 Las Hermanas entered the repertoire of Western Theatre Ballet and in 1971 that of the Royal Ballet New Group. It was most recently performed by Sarasota Ballet in Florida in 2007.