"You can't make 'em drink the water," Jackson told ABC News in an interview airing Wednesday.
When asked if her brother was in denial about his addiction, she replied, "Possibly."
"I wish he could answer this question for you and not me," she told ABC's Robin Roberts. "I felt that he was in denial."
She blames Dr. Conrad Murray, Michael Jackson's personal physician, for his death June 25.
Prosecutors in Los Angeles are weighing charges against Murray, who told police he administered a powerful anesthetic to the singer shortly before he died.
During the interview, the 43-year-old Janet Jackson spoke of her upcoming diet book, "True You," which chronicles her lifelong struggle to control her weight.
She has gained some mastery over her self-image, she said, after years of "just picking yourself apart all the time because you're so used to being kind of picked apart."
She said she hasn't seen "This Is It," the new documentary film spotlighting her late brother as he prepared for the concert tour that would've taken place last summer.
"I definitely won't, not right now," she said. "I don't know if I will ever see it."