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 ménage 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