In a family of vocalists, it was Cleotha Staples' smooth and velvety voice that helped set apart the sound of the influential and best-selling gospel group The Staple Singers.

Staples, the eldest sister and member of the group her father Roebuck "Pops" Staples started in the 1940s, died Thursday at age 78. She was at her Chicago home and had been suffering from Alzheimer's disease for the past decade, said family friend and music publicist Bill Carpenter.

The group included sisters Pervis, Yvonne, Mavis and Cynthia, but Cleotha was the backbone, defining herself by being the "strong, silent type," said Carpenter, author of "Uncloudy Day: The Gospel Music Encyclopedia."

"When she was young they used to call her granny because she acted like a granny in terms of being wise and always sure of the best thing to do," Carpenter said.

Staples, known as "Cleedi," was inducted into the Rock and Roll Hall of Fame with her family in 1999 and received a lifetime achievement award from the Grammys in 2005. The Staple Singers gained a huge audience with their first No. 1 hit "I'll Take You There" in 1972 and followed with top 40 hits "Respect Yourself," ''Heavy Makes You Happy," and "If You're Ready (Come Go With Me)."

The family's music career had its roots with Pops Staples, a manual laborer who strummed a $10 guitar while teaching his children gospel songs to keep them entertained in the evenings. They sang in church one Sunday morning in 1948, and three encores and a heavy church offering basket convinced Pops music was in the family's future.

The Staple Singers was born. Two decades later the group became an unlikely hit maker for the Stax label. The Staple Singers had a string of Top 40 hits with Stax in the late 1960s, earning them the nickname "God's greatest hitmakers."

When the children were younger, it was Cleotha's high voice that influenced Pops Staples' guitar playing and in turn influenced The Staple Singers sound, Carpenter said.

"When Pops used to sit them in a circle and play music with them he was sort of feeding off of her voice," Carpenter said. "It was high in a light way, sort of soothing and velvety so his guitar playing bounced off of that."

Cleotha Staples was born April 11, 1934, in Drew, Miss., the first child of Pops and his wife, Oceola. Two years later, the family moved to Chicago, where Pops worked a variety of jobs performing manual labor and Oceola worked at a hotel. Chicago also was where the family's four other children were born.

Pops and Mavis primarily took the lead on the group's vocals, but a 1969 recording of duets featured Cleotha's voice on the song "It's Too Late," a bluesy ballad about a lost love.

The family also became active in the civil rights movement after hearing the Rev. Martin Luther King Jr. deliver a sermon while they were on tour in Montgomery, Ala., in 1962. They went on to perform at events at King's request. It was during that period that the family began recording protest songs, such as "Freedom Highway," as well as gospel. The group even covered Bob Dylan's "Blowin' in the Wind."

At the end of her life, Cleotha Staples lived near her sisters Mavis and Yvonne on Chicago's South Side. Carpenter said the sisters were vigilant caretakers of Cleotha, just had she had been when the sisters were younger.