Erika Goldring/WireImage; Steve Granitz/WireImage
The Sweet Magnolias actress sat down with the podcast Call Her Daddy for a two-part episode premiering Monday night, telling host Alex Cooper that it was difficult to watch Britney grieve the break-up.
"I think everyone thought it was forever," Spears, 30, said in a preview shared exclusively with PEOPLE. "I was so sad, first off because my sister was so sad, but that, too, was the first time I saw [that] maybe I didn't know everything about the relationship. Perhaps they were protecting me and obviously why would they talk to me about anything."
Britney and Timberlake, both 40, dated from 1998 to 2002 (when Jamie Lynn Spears was in her early teens).
While the star said she "truly" does not know the reason why the couple called it quits, Spears said she sympathized with her sister, especially after Timberlake released the track "Cry Me a River" in 2002.
— Call Her Daddy (@callherdaddy) January 17, 2022
The song, which helped mark his debut as a solo artist following his time in *NSYNC, was largely interpreted as a dig at Britney due to lyrics that implied an ex of Timberlake had cheated.
"I thought how heartbreaking it must have been for my sister when 'Cry Me a River' came out," Spears said. "Don't get me wrong, like, that's a way to launch your solo career, right? That's a way to do it, but I felt really sad. And my sister, she wrote that song 'Everytime' and she's beyond brilliant with anything creative and she picked that song out on the piano and she wrote it and it still makes me cry because I think about how heartbroken she was, because that was her song and that was his song."
Timberlake has since issued a public apology to Britney when last year's New York Times documentary Framing Britney Spears — which featured insight into "Cry Me a River" and a clip of a 2002 interview with radio hosts Star and Buc Wild in which Timberlake admits that he and Britney had sex — brought renewed attention to the end of their relationship.
Dave Hogan/Getty Images Justin Timberlake and Britney Spears
"I am deeply sorry for the times in my life where my actions contributed to the problem, where I spoke out of turn or did not speak up for what was right," Timberlake wrote on social media in February.
"I understand that I fell short in these moments and in many others and benefited from a system that condones misogyny and racism," Timberlake continued in part. "I specifically want to apologize to Britney Spears and Janet Jackson both individually, because I care for and respect these women and I know I failed."
Meanwhile, Jamie Lynn Spears and her older sister have spent recent days embroiled in a war of words in light of the upcoming release of Jamie Lynn's memoir, Things I Should Have Said.
RELATED VIDEO: Jamie Lynn Spears Says She Has Nothing 'But Love & Support' for Sister Britney in New Interview
Though Britney initially accused Jamie Lynn of trying to "sell a book at [her] expense," she later clarified that she loves her younger sister "unconditionally" but is "hurt by the things you are making up about me!!!"
Jamie Lynn wrote on her Instagram Story on Saturday that she wanted Britney to call her so that they could handle the situation "privately like sisters should."
"Britney- Just call me, I have attempted many times to speak to you directly and handle this privately like sisters should, but you still choose to do everything on a public platform," Jamie Lynn wrote.
She added, "In the meantime, please stop continuing the narrative that I haven't been there for you or that I'm making things up. I'm happy to share how many times I've reached out to you, supported you and tried to help you. This is embarrassing and has to stop. I love you."
Image Group LA/getty Britney and Jamie Lynn Spears in 2017
In a recent interview with PEOPLE, Jamie Lynn discussed the end of Britney's 13-year conservatorship in November.
"I didn't have a say when [the conservatorship] happened. My sister was going through a hard time, I was still a child, and that's what everybody, I think, forgets: I was just a kid going through my own crap," said Jamie Lynn.
"My intentions have always been pure when it comes to my family," she added.
The first part of Jamie Lynn Spear's two-part Call Her Daddy interview will premiere on Monday at 10 p.m. ET, and the second will drop on Tuesday at 10 p.m. exclusively on Spotify.