Denise, Dominique and Tanya Brown sat for an interview with Diane Sawyer ahead of a new documentary that's coming out that looks back on Nicole's life and murder -- which they believe was carried out at the hands of O.J. ... even though he was criminally acquitted.

Despite that, the Brown siblings say there were warning signs in Nicole and O.J.'s relationship ... including domestic violence, which they reflect upon in this sit-down and in the doc.

Sadly ... there are more questions than answers all these years later, especially with O.J. dying last month and never having confessed. The Brown sisters are asked how they feel about O.J. being gone ... and it sounds like they're conflicted.

Take a listen ... Dominique says it's confusing and complicated processing O.J.'s passing, especially because she has a relationship with his and Nicole's two children together.

Of course ... we know the kids, who are now adults, were around O.J. in his final days -- so they obviously had some kind of relationship with their father. As we reported ... O.J. died of prostate cancer at his home in Las Vegas. His longtime attorney is sorting out his estate and debts ... including open judgments the Browns and Goldmans have been chasing for years.

The Lifetime doc -- which is called "The Life & Murder of Nicole Brown Simpson" -- happened to be coming together just as the O.J. death news came down ... and it'll be a full retrospective of what led up to Nicole's murder in the summer of '94.