Why is Sister Sledge at the Festival of Families?

Sister Sledge will sing at the papal Parkway concert.

Philadelphia’s own disco queens Sister Sledge are best known for their 1979 hit “We Are a Family.” They learned to sing under the tutelage of their Julliard-educated grandmother, Viola Beatrix Hairston Williams, and started singing under the moniker Mrs. Williams Grandaughters.

Their fame grew in the early 1970s, and they were invited to perform at Zaire ‘74, the three-day music fest that accompanied Rumble in the Jungle, the Muhammed Ali-George Foreman fight. All the while, Debbie, Joni, and Kim made sure to get their college degrees at Temple University (youngest sister Kathy ended her education as a sophomore to go on tour with the group). Their big break came with 1979`s “We Are a Family.” Kathy left the group in 1989, joining her sisters for one-off performances, including President Barack Obama's 2009 inuaguration. Kathy, who took lead vocals on "We Are a Family" when she was only 19, will not perform with her sisters at the Festival of Families. ""I wish my sisters the best of luck," Kathy told the Daily News. "This would've been an amazing platform to express that we are family."

Watch: Sister Sledge, "We Are a Family"