Joni Sledge, one-fourth of Sister Sledge, died from a preexisting medical condition

Law enforcement sources tell us there was no foul play, and the 60-year-old singer's cause of death will be listed as natural. Joni was found dead in her Arizona home on Friday.

A publicist for the disco greats said Joni had not been ill recently.

It's still unclear what the fatal condition was, but we're told an autopsy was not necessary because her doctor signed off on "natural causes."

Joni and her sisters had huge hits in the '70s with songs like "We Are Family" and "The Greatest Dancer."