For all his heroic clutch hitting that won the National League play-offs and the World Series, what Pittsburgh Pirate Willie Stargell was really Most Valuable Player of was the jukebox. Back in June, it was "Pop" Stargell, 38, who spotted We Are Family, the disco hit by Sister Sledge, and turned it into the galvanizing fight song of his fans and team.

Family Fever swept the city. The phrase "The Family" was stenciled on the dugout roof and on signs and T-shirts everywhere. The tune was played mercilessly over the stadium P.A. system, but of course it's easier to get down to than Take Me Out to the Ball Game. When the Pirates clinched the pennant, the players' wives exultantly shook their booties atop the dugout to the horror of baseball traditionalists. But even the Great Explainer, Howard Cosell, never made clear just who Sister Sledge is. For all the 80 million ABC viewers knew, it was some latter-day singing nun. The irony is that Sledge is four sisters—Debbie, 25, Joni, 23, Kim, 22, and Kathie, 21—who were on a three-week tour of Europe at the time; moreover, they are third-generation Philadelphians who, says Kim, "appreciate our Phillies. They should have used our song."

Kathie, who sings lead on Family, says, "It's a miracle. We thought the song had made as much noise as it ever would. Then the Pirates came along. It shows how God can act in mysterious ways." Not the least of them is that Family was also the anthem of the 75,000 strong Gay March on Washington last month as well as of numerous campus sororities. Joni observes, "I can see why. The song is about people leaning on each other for help."

Like the Pirates, the song turned the sisters into winners after two money losing LPs. The We Are Family album and title track have each sold more than a million, and network exposure during the Pirates' 10 post-season games hardly hurt Sledge's thriving road act. They are now booked to play Tonight, a Bob Hope special and, next week, the NBC children's show Hot Hero Sandwich. There is only one hitch—the album was written and produced by Nile Rodgers and Bernard Edwards of the group Chic—and Sister Sledge thinks it's about time to move out on their own. "It's been a good marriage for us," says Debbie, "but there's a point where we have to grow."

Restlessness with a proven formula is rare in music—but understandable chez Sledge. They are hardly the typical puppets of a disco producer. The three older sisters Sledge are Temple University grads, and Kathie interrupted her sophomore year there for the tour. The Sledge melodic roots go back to their music teacher grandmother Viola Williams. Mother Sledge, Flo, who now co-manages the group, was a chorine before marrying tap dancer Edwin Sledge of the Fred and Sledge duo, performers on Broadway and The Ed Sullivan Show. They split in 1964, and Edwin joined the ministry in Houston, raised another family and rarely sees his daughters. So Flo waitressed to support her girls. "Because my hours were long and crazy," she recalls, "they sang to have something to do." The sisters harmonized in front of mirrors before braving church and school appearances, but were prepared for stardom: They sprayed old 45s with gold paint and hung them on the walls.

Just in case, though, the Sledges developed fallback skills. Debbie is an accomplished painter and sculptor whose work adorns the family's modest four-bedroom row house in northeast Philadelphia. She lives with her husband, Bernard Young, art professor at the University of Kentucky, and daughters Amber, 2, and Camille, 5 months, in Lexington, Ky. Kim, a Pan-African major with dreams of a diplomatic career as "a black Shirley Temple Black," lives in Chester, Pa. with husband Bernard Hopewell, a YMCA program director. Kathie, a therapeutic recreation major, and Joni, a communications grad, will soon leave home for their own apartments and "wait for the right men to come along," as Kathie puts it. Then behind the starting lineup, the Sledges have a pinch singer: oldest sister Carol, 27, who subbed for Debbie during her two maternity leaves. (Carol, divorced with a son, Jahmal, 10, is a nursery school director in Philadelphia.)

"Family has given us recognition," says Debbie, and "our next step is to earn respect as true artists." They remain sensibly grounded despite such lofty sights. As Kathie notes wistfully, "One day Sister Sledge will have to go. But," she adds with the wisdom of a Pop Stargell, "the Sledge Family will still be here."