Leolah Brown says she has contacted police in Beverly Hills with her suspicions about the circumstances surrounding the 48-year-old singer's death on February 11.
She said: ''I believe Whitney's death was not accidental. No.''
She refuses to speculate about who she thinks is responsible, but added: ''My attorney has advised me not to get into that at all, but, it's very important to know that she did not just pass away like that. No. And if it's the last thing that I do, I'm going to find out what truly happened to my sister.''
However, she insists her brother is not to blame for the star's demise. She said: ''Bobby has nothing, nothing at all to do with Whitney's death.''
A post-mortem failed to reveal how Whitney - whose lifeless body was found in the bath of her Beverly Hilton Hotel suite - died, and the results of a toxicology report are still outstanding.
A source told the Los Angeles Times that police are still interviewing family, friends and doctors to determine if the prescription DRUGS found in her hotel room played a role in her death.
Meanwhile, Leolah says she is worried about the ''safety'' of Bobby and Whitney's 18-year-old daughter Bobbi Kristina - who she calls Krissy.
She told Access Hollywood: ''I am concerned about my niece Bobbi Kristina, yes.
''Krissy is not safe right now. I want to say to Krissy that it's very, very, very important that she does not trust anybody at this time. Anybody except her father, her grandmother Cissy and myself. And I mean that.
''Right now is the time to just really shut the door.''
While remaining tight-lipped about who could pose a threat to Bobbi - who was hospitalised following her mother's death - Leolah's lawyer Reginald Mason added: ''There are persons that possibly would want to take advantage of Bobbi Kristina's fragile state of mind.''
Notorious British filmmaker Nick Broomfield teams up with Austrian music documentary producer Rudi Dolezal to...
Whitney Houston, the woman behind the 1992 number one hit cover of 'I Will Always...
A movie that preaches dishonesty, trickery and manipulation as the keys to romantic happiness, "Two...