Jamie Lynn Spears says her sister Britney Spears was 'so sad' after splitting ...

Britney Spears was 'so sad' following her split from Justin Timberlake, according to sister Jamie Lynn Spears.

In a sneak peek of Jamie Lynn's upcoming interview with Call Her Daddy obtained by People, Jamie said she still 'cries' remembering how heartbroken her sister was over the ordeal.

'I think everyone thought it was forever,' Jamie Lynn, 30, said. '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.'

Jamie Lynn was only in her early teens when her big sister split up from Justin after four years together in 2002. 

The Zoey 101 actress says she 'truly' does not know why the couple broke up.

But her heart went out to Britney following the release of Timberlake's breakup track Cry Me A River, which all but accused the pop star of cheating on him and included a video of a Britney lookalike reaping the consequences of infidelity.

'I thought how heartbreaking it must have been for my sister when Cry Me a River came out,' she 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.'

Last year Justin published an apology to Britney after the release of the Framing Britney Spears documentary, which explored her conservatorship. She has since been freed from the legal arrangement.

'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,' he posted to Instagram. 

'I understand that I fell short in these moments and in many others and benefited from a system that condones misogyny and racism,' he said. '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.'

Britney's heartbreak wasn't the only topic discussed on the upcoming interview, set to drop Monday at midnight EST.

Jamie Lynn, who has been locked in a feud with her big sister as she prepares to release her tell-all, also claimed to have a text message from Britney which clears her name. 

'You have a text message that clears your name from your sister. Can you pull out your phone and read that recent text from Britney,' host Alexandra Cooper asks. 

Jamie Lynn then asks for her phone and carefully reads the screen before saying something off the microphone. 

After a moment, she speaks into

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