The singer's niece Lauren, who is known as Lolo to friends and family, was found unresponsive in her Maryland home at around noon on Monday.
Her father is Toni's brother Michael Conrad Braxton Jr.
He told TMZ that she died as a result of a heart condition but did not say what that condition is.
Toni, 51, is traveling for work and spent the weekend performing in Trinidad and Tobago. She has not yet commented on her death.
It is unclear if Lauren was alone in her home or if she lives with her father.
Paramedics were called to the property at midday and pronounced her dead on the scene.
Her father is a singer and songwriter, like her famous aunts.
He also appears in the family's reality TV show, Braxton Family Values, with Lauren's sister Ashlee but she does not.