Keira and Sienna on the Wales shore

For Knightley, the film is also a family affair -it was written by her playwright mother Sharman McDonald.

Miller is proving to be a silver screen m¨¦nage a trois specialist of sorts.

She has taken up the role of threesome-loving 1960s figure Louise Ferrier in the film Hippie Hippie Shake, which starts shooting in September.

Prints charming: Keira and Sienna donned floral skirts and cardigans for the shoot

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