Houston's half-brother Gary Garland-Houston and her assistant, Mary Jones, both made the claims against Dee Dee, who died in 2008.
The film, Whitney, is directed by Scottish filmmaker Kevin Macdonald.
Dee Dee Warwick is the younger sister of soul singer Dionne Warwick and was the niece of Houston's mother.
Whitney debuted in Cannes on Wednesday night.
The Warwick family has been contacted for comment by the BBC.
Houston, who sold millions of records and had hits with songs like I Will Always Love You and I Wanna Dance With Somebody, died in 2012 at the age of 48.
She drowned in a bath in a hotel and the coroner ruled that cocaine use and heart disease were factors in her death.
Houston ended her volatile 15-year marriage to singer Bobby Brown in 2007.
Their daughter, Bobbi Kristina Brown, died at a hospice in 2015 at the age of 22, six months after she was found unresponsive in a bath.