"It's just heartbreak," a tearful Roberts, 46, tells WSJ. Magazine in an interview conducted March 1, the day before this year's Academy Awards, but not released until Monday.
"It's only been 20 days. There aren't words to explain what any of us have been through in these last 20 days," she said. "It's hour by hour some days, but you just keep looking ahead."
Motes, a 37-year-old former production assistant, was found dead inside her Los Angeles apartment of an apparent drug overdose on Feb. 9, her family announced the following day.
In the interview, Roberts did not address rumors that Motes left a suicide note criticizing her superstar older sister, and instead focused on the grief surrounding her death.
"You don't want anything bad to happen to anyone, but there are so many tragic, painful, inexplicable things in the world. But [as with] any situation of challenge and despair, we must find a way, as a family," she said. "It's so hard to formulate a sentence about it outside the weepy huddle of my family."
Professionally, Roberts will appear in the May 25 HBO adaptation of the Larry Kramer drama about the early days of the AIDS epidemic, The Normal Heart, in which she plays a medical pioneer with polio helping patients fight the disease as best they could back then.
In her personal life, she is utilizing various methods to cope with her loss, as well as introducing mindfulness to her and husband Danny Moder's children, twins Hazel and Phinnaeus, 9, and Henry, 5.
"Meditation or chanting or any of those things can be so joyous and also very quieting," she explained. "We share and just say, 'This is a way I comfort myself.' "