Britney and Justin dated from 1999 to 2002, and Jamie Lynn said she remembers the former NYSYNC member "fondly."
"That was probably one of the best times in my whole family's life," Jamie Lynn, 30, explained. "Things were good and we were experiencing the good side of things. But I just remember that he was pretty thoughtful... I just felt like he was funny."
Her positive feelings toward Justin meant that she was "so sad" when he and her sister broke up.
"I don't really know why I was so sad, but I was because I think everybody thought it was forever. But I was so sad first off because my sister was so sad," Jamie Lynn said. "That also too was the first time I saw [that] maybe I didn't know everything about the relationship, because perhaps they were protecting me. Obviously, why would they talk to me about anything? It was really sad, though, seeing my sister be so heartbroken."
Though Jamie Lynn noted that she doesn't know what led to Britney and Justin's split, she said that they were "criminalized" and "held to a different standard" than others their age.
The public nature of their breakup, Jamie Lynn said, was made even harder when Justin released "Cry Me a River" in 2002, a song widely thought to be about his split from Britney.
"Looking back with that perspective and seeing how heartbreaking it must've been for my sister when 'Cry Me a River' came out," Jamie Lynn said. "Don't get me wrong, that's a way to launch your solo career, but I felt really sad."
The next year, Britney released her own song, "Everytime," which some believe is her response to "Cry Me a River."
"My sister, she wrote that song 'Everytime,' and she's obviously beyond brilliant when it comes to 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," Jamie Lynn said. "That was his song, but 'Everytime' was her song. It still breaks my heart to think about that."
Following Britney and Justin's breakup, Jamie Lynn claimed that Britney went through a rough time. Things came to a head in 2004, when Britney tied the knot with Jason Alexander in Las Vegas.
Following Britney's short-lived marriage, Jamie Lynn alleged that she went through a terrifying incident with her sister. In her book Jamie Lynn writes, "One time, she said to me, 'Baby, I’m scared,' and took a large knife from the kitchen, pulled me along to my room, and she locked us both inside. She put the knife in the bedside table drawer and simply repeated, 'I’m scared.' She needed me to sleep beside her."
Britney denied the incident in a tweet, writing that she was "never" around Jamie Lynn with a knife and would never "even think to do such."
Jamie Lynn, however, stuck to her story, telling the podcast that she doesn't know what Britney, who's 10 years her senior, was scared of during that alleged incident.
"I was very young. This had come right after the whole marriage in Vegas and stuff. I think that people need to remember, I'm telling the story about what I experienced... that was an experience I went through and it was really scary. I felt really unsafe," Jamie Lynn said. "I think it's important that I say that, because I didn't understand it. I was a kid. I just knew I felt really scared, especially from somebody that [when] I'm scared I run to you, why am I now scared with you?... I never felt like she was scared in a way that she was going to harm me or anything, but it just felt weird. I didn't understand it."
At the time, Jamie Lynn said, she "was not allowed to have feelings about anything," so she "started to really not make a big deal out of things." Now, though, she believes the alleged incident was "really f**ked up" and "not normal."
Jamie Lynn claimed that she told her parents, Jamie and Lynne Spears, after the fact, but that there wasn't any action taken, at least that she saw. ET has reached out to lawyers for Britney, Jamie, and Lynne in regard to Jamie Lynn's book and the allegations contained within.
"I told them about it, and they were all having conversations about how things did seem odd," Jamie Lynn said. "... They asked me to come in there and tell this therapist about it and I did, and then I never heard anything else about it. So then I was like, 'I guess that's just a thing that happened.'
Looking back now, Jamie Lynn said she views her sister as "a brilliant young woman who going through a hard time."
"If she couldn't stand up for herself then somebody should've. I wasn't an adult then. I couldn't. Somebody should've said, 'Stop the f**king presses. Give this girl a f**king minute,'" Jamie Lynn said. "... Why is that not OK to say, 'I need to take a f**king minute and I need to do what's good for my well-being and everyone else's well-being'? Maybe that would've made a big difference for me and my sister."
A few years later, paparazzi caught Britney shaving her head, and photos of it were widely distributed. Jamie Lynn, a teenager at the time, found out about it when her friends showed her the pics online.
"I had been at a basketball game, cheerleading because I was a cheerleader. We got home... and they were like, 'Oh my God, Jamie Lynn. Do you want to see this?' And I was like, 'What do you mean?'" Jamie Lynn recalled. "It almost felt like one of my friends took enjoyment out of it and I didn't like that. I remember being more pissed off about that."
"I was by myself. I didn't have my momma or my daddy. They were there trying to help her. I was by myself and I had to pretend like it didn't bother me," she continued through tears. "... I just wanted to be with [Britney]... All I wanted to do was go home and get in my own bed, but I had to stay at my friend's house... I really don't even know what I said. I can't recall. I kind of just acted like it wasn't that big of a deal, but it was a huge deal."
Jamie Lynn's inability to speak up at that time, and during other instances she details in her book, are a big reason she penned her memoir in the first place. She doesn't regret doing so, even amid the public feud it's sparked with Britney.
"My sister, to this day, is going to always to me be the most bad a** performer that every walked on this Earth. Nothing's going to change that. The problem was, I couldn't be responsible for everything that happened around her, and also be authentic to myself," Jamie Lynn said. "I wasn't allowed to be. I guess I didn't have the courage to do it. I guess I was too scared... I'm breaking a true pattern that has been ingrained in me. It is hard, and I'm scared, and I'm all of those things, but I have to do this."
When ET's Lauren Zima spoke with Jamie Lynn, the actress also shared that she wrote the book for her daughters, Maddie, 13, and Ivey, 3, and for anyone else who has ever felt silenced.
"I wrote this book so that I could show my daughters to use their voice," she told ET. "I hope that tells other people to do the same. Even if you’re afraid, and even if you feel like it’s something that’s scary for you to do, your voice matters. You should always feel like you can control your narrative."