Nicole Brown Simpson's sisters share 'complicated' reaction to OJ Simpson's death ahead of 30-year anniversary of murders

Nicole Brown was 18 years old, working as a waitress and planning to study photography when she met her future husband, O.J. Simpson.

She was brimming with confidence, laughter and generosity, according to her sisters.

But the Brown sisters said the NFL player-turned-movie star lured Nicole Brown into what would become a cycle of domestic abuse.

Watch "Good Morning America" Tuesday and Wednesday at 7 a.m. ET for more from Diane Sawyer's interview with Nicole Brown Simpson's sisters: Denise, Dominique and Tanya.

They married in 1985, and behind the doors of their glamorous house in Los Angeles' Brentwood neighborhood, the sisters now know that Simpson relentlessly criticized his wife -- and that escalated to physical violence.

The family said Simpson's rage continued after the couple's divorce in 1992 and up until Brown Simpson's murder in 1994.

Next month -- June 12 -- will mark 30 years since Brown Simpson, 35, and Ron Goldman, 25, were brutally stabbed to death outside Brown Simpson's home.

Simpson was arrested for the double murder, and in October 1995, after a trial that captivated the nation, he was acquitted of all criminal charges. In 1997, a civil jury found Simpson liable for wrongful death.

The Brown sisters have since worked to bring awareness to domestic violence.

When Nicole Brown Simpson confided in her sister, Denise Brown, about a violent incident with O.J. Simpson, Denise Brown now looks back and thinks she said the wrong thing.

"I asked her all the 'whys' that you don't ask. ... 'Why don't you just get out of this relationship? Why don't you just leave him?'" Denise Brown said. "Those are things that you don't say, and I didn't realize that at the time."

"I just wish that I had known more," Dominique Brown added. "I just wonder if I could have done more to help or to listen to her."

The three sisters saw down for an interview with ABC News anchor Diane Sawyer ahead of a new lifetime documentary, "The Life & Murder of Nicole Brown Simpson," which airs on June 1 and 2.

The Brown sisters have campaigned to strengthen domestic abuse laws, including the Violence Against Women Act, dedicating their work to the sister they miss.

"I sit there and I go, 'Why did it have to take my sister?'" Denise Brown said. "Why did it have to take Nicole for people to understand that domestic violence can kill?"

Simpson, who always maintained his innocence, died of cancer last month at the age of 76.

When Denise Brown learned of O.J. Simpson's death, she said, "I felt, like, this sense of kind of relief in a way."

But the sisters added that his death also brought confusing feelings, because they felt for their niece and nephew, O.J. Simpson and Nicole Brown Simpson's two children, Sydney and Justin.

"It's very complicated. But I have a relationship with the kids that means everything to me, and I was ... very, very sad for them," Dominique Brown said.

Some say among the questions left unanswered with Simpson's death is whether the former Buffalo Bills star suffered from chronic traumatic encephalopathy, commonly known as CTE.

CTE is a degenerative brain disease found in those with a history of repetitive brain trauma and can only be formally diagnosed after an autopsy. CTE has been diagnosed in athletes who've committed acts of violence, including former New England Patriots tight end Aaron Hernandez and former NFL player Phillip Adams.

"I had a conversation with Leigh Steinberg, the sports agent," Tayna Brown said. "And he was talking about, 'There could have been a TBI, traumatic brain injury [with Simpson.]' And I said, 'No. Possibly, but, when you know the person's character and what he's capable of, yeah, that's not -- that's -- that's violence, that's power and control."

As the Browns near the 30-year mark of their sister's murder, they say the grief hasn't let up.

"Sometimes I would think, 'Gosh, I am just staggeringly sad today,'" Dominique Brown said. "And I would realize that it was the day she was murdered, or it was her birthday or something, or some memory would come up."

Tanya Brown, who was 24 years old when her sister was killed, said, "It was rough, unresolved grief. Everything hit me hard 10 years later."

"The pain doesn't go away, it doesn't subside, doesn't get easier," Denise Brown said. "But you do the best you can and you move on. And it's one step forward."

If you or a loved one are experiencing domestic violence, please call the National Domestic Violence Hotline at 1-800-799-7233, or access help through their chat at thehotline.org.