Updated January 25, 1988 12:00 PM

When Robert Harling announced a year ago that he had written a play, his Louisiana parents were dumbfounded. A struggling New York actor with a never-used law degree, their son hadn’t been known to compose anything more creative than a college term paper. Then there was his play’s subject. Just six months after the death of his 32-year-old sister from complications of diabetes, Robert was telling the story of her last years—and transforming it into comedy. “I don’t think my family knew how to react,” admits Harling, 36. “It was sort of like, ‘You’ve written a comedy about that?’ They were very uncomfortable.”

That, however, was then. Steel Magnolias, Harling’s darkly comic drama about six Southern women who meet at their local beauty parlor for chat and emotional sustenance, is an unqualified success. For seven months, it has played off-Broadway to misty-eyed audiences and enthusiastic critics. The movie rights—with Harling as screenwriter—were sold to producer Ray Stark last fall. Liz Taylor has expressed interest in a role. So has Bette Davis, who summoned Harling to tea. (He found her “full of spunk,” he says.) Paramount has signed him to write two screenplays. But best of all, says Harling, his family—father Bob, a retired paper company executive; mother Margaret, a mental health counselor, and brother John, 26, an electrical engineer—are delighted. “They’ve all seen it several times and loved it,” Harling says. “It’s like I’ve been able to take the pieces of the most shattering experience that could happen and somehow make them into one of the most exciting things. There’s great joy in that.”

Growing up in little (pop. 16,664) Natchitoches, La., Harling never conceived of himself as a writer: His dream was to act. He attended Northwestern State University of Louisiana and then Tulane law school “to have something to fall back on.” But when law proved too dry—”not many laughs in Brown vs. the Board of Education”—he skipped his bar exam to take a job in summer stock. Arriving in New York in 1978, he drifted from one unfulfilling bit part to another, hawking everything from Kentucky Fried Chicken to flashlights in TV commercials, which was the closest he came to a break.

But there were graver problems back home in Natchitoches. Harling’s married sister, pediatric nurse Susan Robinson, had decided, against her doctors’ advice, to have a baby. “Diabetics have healthy babies all the time, but she was a little more brittle than most,” says Harling. “The doctors said it would put too much strain on her vascular system and kidneys.” Still, Susan chose to experience “15 minutes of wonderful instead of a lifetime of nothing special,” as her counterpart, Shelby, puts it in Magnolias. On February 23, 1983, she delivered a healthy son, Robert. A year later, her kidneys failed, and a transplant donated by her mother failed to function. In October 1985, Susan died.

Robert was devastated, and furious at fate. “I ranted and raved to a playwright friend, and he just kept saying, ‘Write it down,’ ” he says. So he did. Magnolias began as a short story, something to provide his nephew and namesake with “a sense of his past”—and then it grew. It is a comedy, Harling says, because “even in the darkest hours there’s stuff that makes you giggle. That’s what my sister would like more than anything—to know that we were able to laugh.”

With the success of Magnolias, Harling, who lives alone in a Manhattan West Side apartment, can bid farewell to his days of hustling for commercials. That, too, he believes, would please Susan. “She was the fixer in the family,” he says. “She was always frustrated that she couldn’t help my career.” And though he is openly awed by his swift success, he has no intention of going Hollywood. “Most of my work will be character-motivated,” he says. “You’re not going to find many car chases or spaceships. Just people living life and getting through it, like they do where I come from. I have a real sense of responsibility about that. Through my sister’s story, I’ve been given the chance to give back, to say Thank you.’ ”