Bobby Brown's sister has broken her silence following the death of her brother's ex-wife Whitney Houston in an effort to warn her niece to watch out for untrustworthy acquaintances.

Leolah Brown fears Bobbi Kristina's fragile state of mind will leave her open to dubious people in her late mother's inner circle.

Appearing on news show Access Hollywood, Brown says, "I am concerned about my niece. Krissi is not safe right now... It's very, very, very important that she does not trust anybody at this time except her father, her grandmother Cissie and myself - and I mean that. Right now is the time to just really shut the door."

Bobby's sister, who was very close to Houston, reveals she has reached out to Los Angeles detectives investigating the superstar's death to make sure they find the truth.

She adds, "I believe Whitney's death was not accidental... My attorney has advised me not to get into that at all but it's very important to know she did not just pass away like that and if it's the last thing that I do, I'm going to find out what happened to my sister."