"We shouldn't be here at all," Vy Higginsen is saying. "A first-time writer, a first-time producer, a first-time director. We should never have gone past three weeks."

Her brother and sister -- who star with her in the musical she and her husband Ken Wydro wrote, produced and directed -- are nodding proudly. "Mama, I Want to Sing" is performed in a long-abandoned Harlem theater with a hand-painted marquee -- about as off-off-Broadway as a New York theater can be. It has no advertising budget to speak of, played weekends for months before any major critic came to see it and features an energetic cast of -- except for Higginsen herself, a local radio personality -- virtual unknowns.

Taking a flier with their own savings, Higginsen and Wydro opened "Mama" for three weeks in March 1983. He figures 300,000 people, most of them arriving in busloads from as far off as Boston and Decatur, Ga., have seen it since. Two hundred buses have come from Washington alone, where the couple is scouting theaters for its first tour, slated to debut in late spring. Fans fill the house for nine shows weekly and jubilantly stand to join the cast in "This Little Light of Mine" at the end. They buy "Mama" sweat shirts and key rings, and demand that New York radio stations play the single the show's producers have pressed.

"We proved you don't need a review to be a success," Wydro says.

"We didn't know the rules, so they really didn't apply," Higginsen adds. In the company's offices above the darkened Heckscher Theatre, she is exulting about plans for a second-year anniversary bash ("with a klieg light and a red carpet"), while her older sister Doris Troy and her brother Randy Higginson join in the good-natured self-congratulation.

"And we didn't ask anybody's permission," Doris puts in.

"People didn't have to look at The New York Times to see if it was good or bad," says Vy, who changed the spelling of her last name when she became a radio personality. "What mattered was the people who sat in the seats. Each one told one and it spread by word of mouth, like a drum."

"Mama, I Want to Sing" is the relentlessly upbeat story of the Higginsons themselves, of Doris' transformation from gospel soloist in their father's Harlem church to a pop singer and songwriter. Her 1963 hit "Just One Look" still packs such oomph that everyone from Martha and the Vandellas to the Hollies and Linda Ronstadt (more than 80 artists in all) has recorded it, that Mazda used it to sell cars for two years, that the current movie "The Flamingo Kid" uses it to embody '60s soul.

Doris, the oldest of the Higginson children ("just say I'm over 21"), still gets royalty checks from the song. "Sometimes nice-sized, sometimes nothing to talk about," she says. "Records had all these strings, this production then. Our little record just had four musicians -- my partner and I overdubbed the voices so we could have three-part harmony. And we had a hit."

"I was going to Roosevelt High School and I was a celebrity because my sister had a hit," says Vy, the youngest, in her late thirties. Her graduation gift was a ticket to Europe, where Doris was touring and had hooked up with the Beatles' Apple Records. "We got off the plane and there was a red carpet and a fan club," she recalls. "It was grand and glorious, changed my whole life."

"I always said Vy was the genius of the family," Doris says fondly.

"I always said Doris was," says Randy. "But you can't mess with any one of us."

The Higginsons were a trio once before. "We'd sing in Daddy's church when we were kids," Doris says. "I could play three chords on the piano and we'd gather around and practice." But they moved in different directions, Doris to Las Vegas to play the Sahara and the "Trop" and Tony Roma's. "Singers don't need hit records to work in Vegas," she points out. Randy, turned fitness buff, teaches skating and skiing and runs a roller-disco dance company.

Vy's smoky voice became a fixture on a New York radio station, and she published a magazine and hosted cable television shows. And one afternoon, walking a beach in Jamaica with her husband, she began recounting Doris' story. "I was telling him about my sister, how she inspired me, and we began to put the story together right then and there. It's not only Doris' story, it's an American story, many people's story. You hear it over and over when you talk to entertainers: 'My father was a minister, my mother led the choir, my aunt played the piano.' "

Indeed, Doris' story is a familiar saga, not only the stuff of "Dreamgirls," but of "The Jazz Singer." There is the priestly father -- Randolph Higginson was pastor at Mount Calvary Pentecostal Church at 130th Street and Lennox Avenue -- played by Randy. "He was a giant in the community," Doris says of their father. "He was The Man. He could get people out of jail. Winos would get off the curb and say, 'Reverend Higginson.' Even though I went out in the world, I carried the things I learned at home with me, how to behave, how to be respectable."

There is the ambitious daughter, between choir practices, sneaking out to the Apollo Theater to hear Billie Holiday and Lena Horne and facing her mother's wrath. "All she had to do was give you that look," says Doris, who plays her mother in the production. (Desiree Coleman, an 18-year-old sensation with a multioctave range, plays Doris.) "I wanted to be out in the world, but our world was the church. We were there all the time. I didn't go to a movie until I was 16."

There's the predictable struggle -- does Doris defy her mother? -- and reconciliation. Geraldine Higginson, widowed and still living in the Bronx, is frequently in the audience. "She still tells us if it's good or not," Randy says. "She wants more God in it."

But Vy supplied an unconventional outcome: Instead of suffering the torments of fame, as show biz plots dictate, the heroine of "Mama" behaves herself and stays clean. "Everybody said, 'How can you have a play without the pitfalls, the violence, the ugly?' " she says. "We proved people can be entertained without hate or drugs. I've interviewed scores of famous people who didn't fall into that -- they managed to stay on the path. Many did have firm foundations they never forgot. It's important that we see that."

In "Mama," Doris even comes back to Harlem to open day-care centers and schools. This last is a bit of wishful thinking, Doris acknowledges. "They're things we'd like to do."

Having passed the break-even point in its ninth month, REACH (the Higginsen-Wydro production company that employs 85, including another sister and several cousins) does have plans. A cast album. Other scripts. A rejuvenated recording career for Doris, who's had a few club dates and a small soap opera role since returning to New York.

And since "Mama" has no dialogue to speak of, relying on Vy as a disc jockey and narrator and on the sheer vocal firepower of its choir and cast, there is no reason it can't have an international life.

"We think we can run for 20 years," Wydro muses, envisioning "The Fantasticks" of Harlem. "And we'll open in Philadelphia, Washington, Chicago, Detroit, San Francisco."

"I want to go to Tokyo," says Doris, who's working on a new album that REACH may produce.

"A European tour," adds Vy, "and the Caribbean and the Third World. Let everyone see the energy and spirit of American music. And it can and should be a movie."

"We'll do Johnny Carson next," says Doris, who has already done Merv Griffin.

"It's wild," Randy says, shaking his head.

"A total trip," Doris agrees.