"God help the mister who comes between me and my sister," runs the song, and there could be a double meaning in that lyric in this fierce, unflinching exposure (sic) of real-life sibling sexuality. Amy and Rosanna Cade first present themselves as pole dancers in a nightclub.
But then the wigs, and the gloves, are off, as fulsome, luxuriant Amy describes her life, and liberation, as a sex worker, and little shaven-haired sis Ros litanises the joys of lesbianism, her worries about family and young brother, her pleasures of the flesh in baths and public places; Amy, too, has pleasured a chap in a railway carriage. Neither sees sex as anything except jolly good fun.
I'm not sure about the naked truth of all this, or the ease with which it's broadcast. Neither woman has children, and we see home movie footage of them both as angelic tiny tots, playing together in the soap suds, running round a park or garden, beautiful both in bubbles and blonde curly hair.
Are they saying there's a seamless growing up in their lives, as though the sexualisation of children is perfectly natural, as it has been, apparently, for them? This is certainly food for thought, though the thinking will probably only start when you've left the theatre and its rather charming, inoffensive flaunting of pudenda, bottoms and all intimate cavities.
All that's missing really is great writing, better dancing and perhaps a song or two more to oil the wheels and anything else that helps make the sex chat show go with more of a swinger.
Sister runs at Summerhall until 24 August
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