Freud would have had a field day. You’d think it was enough for the Citizens’ theatre to put on a musical adaptation of What Ever Happened to Baby Jane?, one of the darkest — and campest — studies of sibling rivalry ever put on screen.

But the Glasgow company has gone one step further by casting real-life brother and sister Julie and Johnny Austin as the married Mr and Mrs Hudson, the parents of the child star Baby Jane and her talented sister Blanche.

“I decided before rehearsals that they’d definitely not had a conjugal relationship since the children were born,” jokes Julie, 29, smiling at her 26-year-old brother over a lunchtime sandwich in the theatre’s rehearsal room. “I’ve decided they’re austere, Midwestern people — for the children, no kissing.”

“They don’t even hold hands,” adds Johnny.

The Citz musical is based closely on Robert Aldrich’s 1962 cult classic melodrama in which Joan Crawford and Bette Davis — who were considered well past their Hollywood prime by then — were brought together to play ageing sisters; one crippled, the other psychotic. In later life Baby Jane is an alcoholic who was briefly popular as a child star, but never talented. She is still trying to get over her fall from grace, deeply resenting her wheelchair-bound sister’s subsequent movie career and taking out her jealousy in a series of increasingly gruesome and violent acts.

In the Glasgow production, the Austins play the girls’ parents in the first section of the show, taking other parts as the action develops. As well as acting, Julie is also the choreographer, and on the day we meet she has spent the morning guiding Johnny beat by beat through a lewd sex act with Andrea Miller’s ageing Baby Jane.

“It’s a very camp, Vegas-style number,” says Julie, describing the libidinous moves in the second-half dance routine. “It’s the sort of conversation I thought I’d never have with my brother.”

But in all the world’s theatrical dynasties, no two actors can ever have seemed less like a Freudian case study. Julie and Johnny are secure, uncompetitive and as comfortable in each other’s company as old friends. Far from being rivals, they are instinctively protective.

“As a family we are very close and we support each other,” says Johnny. “Working in something your family is involved in means you can keep that closeness because you have a shared language. We’ve been lucky to be able to keep those relationships strong.”

As they describe their upbringing in Glasgow and Paisley, living with their bank manager mother, fireman brother Paul, 24, and 13-year-old sister Jane, it suddenly strikes them how idyllic it sounds.

“We speak to each other every day,” says Julie, wife of the award-winning Hollywood screenwriter Ed McCardie. “That sounds a bit like The Waltons, doesn’t it? I don’t know if that’s freaky or not — maybe we’re just too nosy about what everyone else is up to.”

Encouraged by her parents to follow her passions, Julie took dance classes from the age of two, moving on to a drama group and graduating to Scottish Youth Theatre. “My mum encouraged us to do all these activities so that when we hit the drinking-cider-on-a-street-corner stage, we’d be too busy to notice it,” she says, admitting that their teenage years were relatively trouble-free.

An authoritative, sometimes scary, presence on stage, Julie’s professional career has included powerful performances in the Royal Lyceum’s Sleeping Beauty, the Citizens’ Vernon God Little and the Tron’s Love Freaks. On screen she has appeared in The Crow Road, The Magdalene Sisters and Braveheart.

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