On stage, they say it's all guts and soul, glamor and glitter. The four Sisters Sledge, dressed in vampy black jump suits and boots, project power and seduction. "We're all-Americans girls," they sing. It's hard to believe.

But after the performance, after they remove the heavy makeup and put on jeans and simple cotton sweaters, there is a dramatic change -- in much more than appearance. They sit shyly, pulling on their hair as they speak. They giggle nervously. These are the same women who, in their act earlier Tuesday evenng for the Delta Sigma Theta sorority convention, rubbed their bodies suggestively against male members of the audience. These are the same women who had men in the front rows of the International Ballroom of the Washington Hilton screaming, "Come on, baby, shake it some more." And they did.

"It's like we live two lives," says Joni, 24. "There are the sisters who entertain and then there are the sisters who are just people. Just regular folks." Sure, it's sex on stage, but at home they're Christians. In conversation, they thank God several times for their family and their success.

The sisters became overnight celebrities in 1979 when sororities and clubs across the country adopted "We Are Family" as their theme song. The Pittsburgh sports teams made it their rallying cheer. Yet long before the disco hit, when the sisters were growing up in Philadelphia, they were "normal students" during the days, professional performers nights and weekends.

"Since childhood, we've dealing with this dual life," says Joni. "Having this schizophrenic life style gets kind of tough. It was especially hard in school. Concerts in Japan one week. Midterm exams the next."

Kathy, 22, and Debbie, 26, nod in agreement. Despite the hectice schedule they maintained as adolescents, they all received scholarships to Temple University. "We knew we couldn't afford to go to college otherwise," says Joni. "We weren't making all that much money. We were doing it for fun."

They had been encouraged to sing from a young age by their grandmother, a former opera singer, who lived with the family. "She gave us an appreciation for all types of music. We got into everything from opera to rock 'n' roll," says Debbie. Then, in 1964, their parents divorced, and singing and writing music became something to do while their mother was at work. They began performing at school socials, teas and church benefits. "We were on our way from there," says Kim, 25. "We just didn't think it would happen so fast. 'We Are Family' drew incredible response."

That's because the song was inspired from the heart, Joni explains. "We really do have a closeness, a special sense of family. It's not an act at all. There is a bond, an inexplicable strength between us that we want to convey in our music."

Meanwhile, their social lives have suffered. "Romance has always been a problem for us," says Joni. "For me, it's because of my independence. I'm used to carryng the burden myself, or with my sisters. With a man, it's hard to relax and let him have some of the control."

Debbie is married to a University of Kentucky professor and lives in Lexington with her three children. On shorter engagements, she takes them on the road with her.They know all the songs, Kathy says, and for a few minutes, they all talk about "the babies." One wouldn't know who the mother was. They share the pride of the first words and the first steps, as they do everything else.

The youngest sister, Kathy, recently married their band's drummer, Philip Lightfoot. "Before the show, he was telling me how pretty Debbie's hair looked," Kathy says with a laugh. "I slapped him and told him never to come between me and my sisters."

There have never been any feelings of jealousy or rivalry between the sisters, Debbie says. "Yes, there are moments of tension," she adds. "It passes quickly. We talk everything out, like who should do the leads on certain songs. We're equals. No one dominates."

When they're not working, Kathy and Kim live in Philadelphia. Joni moved to Los Angeles to pursue an acting career. "We're so used to each other," says Kim. "As soon as we're apart, we're on the phone. We have friends, but no one understands us like we do."

As teen-agers, they would sometimes come home at 4 a.m. ahd hve to be at the bus stop at 7. They would tell friends they were tired from work, and the other students would respond, "Oh, you call what you do work?" "I would tell people about some of our shows and I don't even think they believed us," says Kim. "Africa? Right."

That deep understanding of one another surfaces on more serious levels, they say. When one of them is depressed, the others can sense it, even if they are in different cities. Often, one will begin a sentence, and another sister will complete it. "Five bodies, one soul, I guess you'd say," says Joni. The fifth is Carol, the oldest sister who is a schollteacher and sometimes substitutes when one of the sister's can't perform.

Perhaps the most important thing they have in common is no apparent craving for fame and money. They want to "be happy, make others happy, and find spiritual fulfillment."

"Oh, we also have a lot of stored-up creativity," Joni adds. "We wnat to use our energies to the fullest."

After the release of their last album, "American Girls," the sisters decided to produce their own material, as well as other groups'. They also are exploring the possibilities of television specials and solo albums. "Working on our own would strengthen us, rather than weaken us," Joni says. "We have to each decide what our goals are and how we're going to get there. If it means making little sacrifices, that's what we'll do." But the group, the family, still comes first, she says.

Debbie rubs her eyes. They're getting tired. All that moving around on stage is pretty draining. "It takes a lot out of you," Kim says."We try to give so much love to the audience. You get back everything that you give, too."

One mor thing says Jone."It's a line from 'We Are Family.' 'Have faith in you and all the things that you do and you won't go wrong. '"

"And God to that," says Debbie, who has "the best bump this side of China," according to one n a group of several male autograph seekers. "Have faith in you and God ."

"Yeah, that's right," Joni replies. The sisters hold hands and squeeze tightly.