Transcript for Whitney Houston's brother, sister-in-law on shocking revelations in new documentary
??? And I will always love you ??? Reporter: The magical voice that gave us chills with "I will always love you." And slayed with "How will I know?" ??? How will I know if he really loves me ??? Reporter: Now Whitney Houston as you never saw her in life. Intimate. There were times when I would look up to god and go, why is this happening to me? Reporter: Raw. I think Paula Abdul, . That girl is singing off key on the record -- Reporter: Real. The explosive new documentary "Whitney" out July 6th taking an unflinching look at the musical icon's tumultuous life, including drug addiction with never before seen home movies and blunt, stunning revelations from her tight inner circle. There were always a lot of secrets. You don't resolve things and deal with things -- they never go away. I'm wondering if there's something that's like a release. I've been waiting to exhale for such a long time. Reporter: Gary Houston speaking exclusively with "Nightline" along with wife pat. Whitney's former manager and executive producer of the documentary. Perhaps the film's biggest revelation, Whitney was allegedly sexually molested as a child. Pat says Whitney never told her who abused her, but that she did talk about the alleged experience. Did you know? We talked about it. But it was referencing my daughter. You know, we would travel with work, we'd argue about me leaving reya behind. I never understood why she was on adamant about being on the road for us. She talked very briefly about her situation, and I understood at that point, and just let it go. I suspected there was some kind of trauma in her childhood just because of the way she acted, the way she talked in interviews about her childhood, how she felt about children, how she wanted to protect children. ??? The children are our future ??? Reporter: Gary says he was also abused. And in the film, director Kevin McDonald says Gary named the abuser of his cousin, Deedee Warwick, sister of Dionne Warwick. I was 8, 9 years old, being fondled. It was a situation I didn't understand. I knew it was abnormal. Did you know Whitney had been molested? I had no idea. Reporter: Long-time assistant Mary Jones says Whitney told her Deedee molested her. She was never able to talk about it to her mother or the professional who was trying to help her, perhaps with her addiction. She did a lot of therapy but she would never open up about this to anyone. Reporter: The film also revealing the shocking extent of the drug abuse by the chart-topping legend. Her drug of choice was? She used cocaine. And she drank. Reporter: The family's roots began on the mean streets of Newark, New Jersey. Gary says hard drugs were part of his life from an early age. I've been turned on to drugs at 10 years old, heroin at 10 years old. 10 years old? Yeah. I did drugs with Whitney from time to time but we were in a different room. I never did with her, me and her together, because it would kind of separate us, distance us. Reporter: Recreational use of drugs part of the rock 'N' roll lifestyle, eventually trapped Whitney in a vise. Something she didn't seem quite able to admit when talking with ABC news' Diane sawyer. Do you think of yourself as an addict? I don't like to think of myself addicted. I like to think of, I had a bad habit. Reporter: But the effects of her bad habit would be on full display at the 2001 Michael Jackson tribute on CBS. ?????? She was very thin. I was very, very concerned. That was when you knew she was in deep trouble? I knew that she was in deep trouble. What did you feel for your sister when you saw what the rest of the world saw? We said, wow, this really has gone overboard. It's really taken the best. Moved into a place where it would be harder to get back to her center. Reporter: Whitney defensive about the performance. I'm not sick, Diane. I am not sick. Let's get that straight. I'm not sick. Okay? I've always been a thin girl. But that's not just thin. No, it isn't. Diane, tell me, do you know? It's scary thin. I can believe what you feel. I can believe that. But do you really know? Do you really know? It's maybe the only interview she gave in her whole life where you feel she's totally present. And she's really meaning everything that she's saying. She's trying to be honest. Reporter: Her drug use may have had the greatest impact on per daughter. Bobbi Kristina. Those closest to Whitney says as a mom she often came up short. What was she most disappointed about? Well, that she could have been a better mother. ??? And mommy loves you girl ??? Reporter: They did have tender moments like this one from the documentary when Bobbi Kristina was just 4. ??? Don't worry ??? Reporter: Shuffled between life on tour with her famous parents and often staying back with family and friends for long stretches. Bobbi Kristina would never know a Normal childhood. Ultimately, you know, Whitney and Bobby made the decisions for their kid. There's only so much anyone else can do. Would you ever have fathomed you would lose Whitney and then three years later lose Bobbi Kristina? It was really, really difficult. I always said if she were 17 years old, we could have gotten her and put her in our home to be with us. But she was 18. Reporter: Like her mother, 22-year-old Bobbi Kristina would be found facedown in a bathtub. And later died. The singer also had a complicated relationship with mom sissy, a famous gospel singer who groomed her daughter from an early age. In one intimate moment, we see their love and closeness. Whitney never forgot who to call on when she needed them. She would always address, I need my mother. So when the chips were down, oh, yeah, she called for her mom. Reporter: One of Whitney's closest confidants was creative director Robby Crawford, a woman her family wanted Whitney to avoid at all costs. What was her relationship with robin? What I saw in Whitney and robin were two best friends. Reporter: Mom sisi addressed the friendship with Oprah. Do you believe that Whitney and robin were in a gay relationship? I don't really know. Would it have bothered you if your daughter, Whitney, was gay? Absolutely. I got a chance to speak to my sister about her before it even got to where the friendship even turned into what it turned into. I've never seen them, you know, in any kind of intimacy kind of situation. But I knew that it was heading somewhere that I didn't really want it. Reporter: The one person she would publicly commit to and go on to marry was of course Bobbi brown. The documentary showing the passion and dysfunction of their marriage up close. When you got people in the media talking about Mr. Houston instead of Mr. Brown, instead of saying like I say, . He let it get to him. Can I ask you, Mrs. Houston? Mrs. Brown. Mrs. Houston-brown? Reporter: No one could save Whitney from her demons as she went in and out of rehab. Do you think she was happy? I thought -- happiness did exist around her. Reporter: This haunting conversation with Diane sawyer taking on a deeper meaning after Whitney's tragic death. If you had to name the devil for you? The biggest devil among them? That would be me. So the biggest devil is me. I'm either my best friend or my worst enemy. And that's how I can deal with it. Ten years from now, give me the perfect life for Whitney Houston. Retired. Sitting, looking at my daughter grow up, become a great woman of god. Grandchildren. Reporter: But ten years later, the night before the grammys, Whitney would be found dead in her Beverly hills hotel room facedown in a water-filled bathtub with drug paraphernalia nearby. We did what we could, but ultimately it was Whitney that needed to decide if she wanted to live or die. How much do you miss her? Tremendously. I miss my sister, and I think she's in a better place. Reporter: For "Nightline," I'm Deborah Roberts in New York.
This transcript has been automatically generated and may not be 100% accurate.