The former Dallas Cowboy will be inducted into the NFL Hall of Fame in Canton, Ohio, almost 30 years after he was eligible. He died in 2002.
Lucille Hester has been speaking for him at NFL events, introducing herself as Hayes' sister.
"The first time I met her was a week before he (Hayes) died at the hospital." said Westine Lodge, Hayes' daughter.
Lodge lives in Jacksonville, Fla., where Hayes grew up.
"Even my Aunt Lena (Hayes' sister) doesn't know who she is," she said.
"She says she's his sister, but I'm not sure," said Lena Johnson, Hayes' sister, who also lives in Florida.
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Hester's name was not mentioned in Hayes' obituary when he died in 2002.
In Garland, Hayes' ex-wife, Janice Hayes-Mohl, said Hester and Hayes were childhood friends who grew up in the same Jacksonville neighborhood.
"He introduced her as his 'play sister,'" Hayes-Mohl said. "He never said she was his sister."
Hayes-Mohl said the family was surprised to learn about the Hall of Fame announcment when they saw it -- and Hester -- on TV.
"She hasn't been very inclusive with the family," Hayes-Mohl said. "It's just been her."
Speaking by phone on the Michael Irvin show on ESPN Radio in Dallas, Hester insisted she is Hayes' half-sister.
She said she shared the same father as Hayes, but had a different mother. When asked who her mother was, Hester mentioned a woman and said, "I think," and said she was adopted by another family.
"I think she's trying to pull a fast one," said Ted McIntosh, Hayes' best friend and former business manager. "I never heard her name or saw her until the week of the funeral."
He also said he has sent a letter to Dallas Cowboys owner Jerry Jones to say that Hester doesn't speak for the family.
The allegations first surfaced Sunday when McIntosh was a guest on the Ben and Skin show on Dallas radio station The Fan 105.3.
Blogger Mike Fisher then examined the controversy in-depth on his Web site, dallasbasketball.com.