Sledge was found unresponsive at her home in Phoenix on Friday and the cause of death has not been determined, according to a statement from the band's publicists.
The Philadelphia-born Joni Sledge, along with her sisters Debbie, Kim and Kathy, formed the group Sister Sledge in 1971.
Aside from "We Are Family," which reached No. 2 on the Billboard Hot 100 and became an international hit, the group also was known for "He's the Greatest Dancer," "Lost in the Music" and a cover of the Motown classic "My Guy."
Joni Sledge did not normally sing lead in the group, but she did on one popular song, "Reach Your Peak."
Sister Sledge performed before Pope Francis in 2015 when he visited Philadelphia.
"Yesterday, numbness fell upon our family," the Sledge Family said in a statement. "We are saddened to inform you that our dear sister, mother, aunt, niece and cousin, Joni passed away yesterday."