"I think it's important that you grieve and feel it," he said. "No matter if you're famous or not, I think having an event and having movies and pictures and feeling that is a good thing. It's part of the process of grieving."
"There's a freeing sense to all of that," he continued. "At the same time, it's obviously a huge loss and they were my girls."
Todd reflected on the heartbreaking moment when Carrie died after going into cardiac arrest on a plane at the age of 60. He told ET that his mother actually asked him for "permission" to "go" just a few hours after Carrie's death.
"My mother said to me the night that Carrie died ... we were talking that night and I didn't know what was happening, but she was setting me up for her leaving the planet," he remembered. "And she really just said to me, 'You know, I know that was really hard on you losing your sister.' [And I said], 'I mean, Carrie was there holding my hand when I took my first steps. I have film of that and it will be in the memorial.' And when I was editing that, it ripped my heart out. But my mother said, 'I know that's tough. It might be tougher even, you know… I don't know for sure when I [will] go.'"
"I was like, 'Well, look, fortunately we don't have to face that right now, you know, we just face this,'" he continued. "But then she was like asking my permission [to go]. I [said], 'You're going to be OK, everything is under control.' And she starts running me back through her estate, which we had done many times. [She's like], 'You're going to take my dog and, you know, all these little instructions.' And [she] even [went through] last-minute changes due to the fact that Carrie left first, as far as the memorial service."
"I thought that we were just going through these events just like one would go back through an estate," Todd added. "But instead the next morning, we had a little further dialogue and she chose to leave the planet in front of my face two feet away. If you had told me this story and I wasn't there, I would have a very hard time believing what I saw. But she literally looked at me and said, 'I want to go be with Carrie' and closed her eyes and went to sleep."
According to Todd, Debbie made it clear in her estate that she didn't "want a big deal" of a service, and asked that she be cremated when the time came. But the night before she died as a result of a stroke, the former Singin' In the Rain star, 84, "changed the whole game."
"She no longer wanted to be cremated, she wanted to be buried with Carrie," he explained. "Now she wanted the tomb idea that I had brought up long ago and got sunk. 'No, I don't want a big tomb, I don't want to draw any attention to myself,' [she said], but now with Carrie gone she said, 'OK.' So we bought this great tomb over at Forest Lawn [cemetery]. She changed her mind that night."
Todd said the doctors have an actual "fancy, medical term" for situations like this, but he can't make up what he witnessed that night while talking with his mom.
"I did not actually say that," he confirmed. "The news said she died of a broken heart. I actually contradicted it. I said, 'No, that is not true. She didn't die of a broken heart, she went to be with Carrie. She said those words to me.'"
Todd further broke down that last conversation he had with his mother, telling ET he remembered "her little forehead was crinkled," something she would do "when she was worried."
"I said, 'Whats up?'" he recalled. "She said, 'You know I really love Carrie.' And I said, 'Of course, I understand that.' She said, 'I don't mean I do not love you.' I said, 'You don't need to say that, I get it! You know, you love in different degrees. You don't need to say that.'"
"And she said, 'I really want to be with her,'" he continued. "Those are all the words that were spoken and she closed her eyes. So, she did what she wanted to do. That wasn't like a person that was sitting there saying, 'My heart is broken.' It was not some scene from Macbeth. She truly just understood what happened."
"It wasn't a mystery to her," he added. "In fact, I'm the one that was crying when I came back from the hospital, not her. She was like, 'Well, you know, I hope that it had gone a different way, but now that it's gone a different way, I'll go be with her.'"
Later in our interview, Todd told ET that to this day, he's "really OK" with "Debbie's exit," but "not so OK" with Carrie's.
"Carrie was in the middle of what was, I thought, her finest hours," he explained. "Her creativity was peaking, the Star Wars thing was obviously back in spades. Everything that she was doing was turning to gold."
The public memorial service for Debbie and Carrie will be live-streamed on DebbieReynolds.com on Saturday March, 25 at 1 p.m. PDT.