Singer Cleotha Staples, founding member of soul-gospel family act the Staple Singers, died on Thursday at her home in Chicago after a long bout with Alzheimer’s. She was 78.

Though younger sister Mavis was the star of the venerable and hugely versatile group, “Cleedy’s” soprano voice contributed an indispensible element to the group’s sound, which evolved over the decades from straight gospel to incorporate soul, folk and R&B, hitting critical mass in the 1970s with a number of huge hits.

As Mavis said of her sister in a statement, “A lot of singers would try to sing like her. Gladys Knight’s background singer, William (Guest), would tell Cleedy, ‘I’m trying to sound like you.’ Her voice would just ring in your ear. It wasn’t harsh or hitting you hard, it was soothing.”

Born in Mississippi, Cleotha was the oldest child of Roebuck “Pops” and Oceola Staples, who soon moved the family to Chicago where brother Pervis and sisters Yvonne and Mavis were born. Pops began assembling his family into a singing act in 1948, playing in churches all over Chicago.

The group recorded its first sides in 1953 for the United label, then moved to Vee-Jay Records, for which 1957’s “Uncloudy Day” notched the group its first gospel circuit hit. In the 1960s the group turned toward politics, performing at rallies alongside Martin Luther King Jr., and covering protest songs such as Bob Dylan’s “Blowin’ in the Wind” and Buffalo Springfield’s “For What It’s Worth.”

Signing with Stax in the late ’60s, the group began moving toward a harder, funkier sound courtesy of producer Al Bell and ace backing from Booker T. and the MGs and the Muscle Shoals Rhythm Section. 1971 saw the group notch its first Top 20 hit with the fiery “Respect Yourself,” while it summated the charts the following year with “I’ll Take You There.” Signing to Curtis Mayfield’s Curtom Records in ’75, the group cut another No. 1 hit with the sublimely groovy Mayfield-penned “Let’s Do It Again”; Cleotha’s wordless high-pitched flourishes on the recording would resurface decades later as a popular sample.

The group appeared in the Martin Scorsese-directed “The Last Waltz” in 1976, and subsequently sank a bit under the mainstream radar, though continuing to perform and record throughout the 1980s.

With the Staple Singers, Cleotha Staples was inducted into the Rock and Roll Hall of Fame in 1999, and received a Grammy lifetime achievement award in 2005.