Jamie Lynn Spears Says Daughter Maddie, 13, Tried to 'Mediate' Incident Between Her and Britney

Jamie Lynn Spears is continuing to open up about her fractured relationship with her sister Britney.

In an interview with Juju Chang that aired Wednesday night on Nightline, the Things I Should Have Said author, 30, described an incident with her pop superstar sibling that she says involved her now 13-year-old daughter Maddie.

While all quarantining together earlier in the pandemic, Jamie Lynn says Britney "cursed" at her and "got in her face" while she was holding her 3-year-old daughter, Ivey.

"I just walked out of the rooms. I was like, we're just all family under one roof. I'm not trying to argue," Jamie Lynn told Chang. "And when I try to remove myself from the situation, that's when I guess she just got really angry and my oldest daughter tried to, you know, mediate the situation."

"I hated that, like, that's not what you're supposed to do. You're 12," added the Sweet Magnolias star, who says her parents, Lynne and Jamie Spears, told her she "shouldn't be upset" and to not "upset Britney."

"I said, 'You just saw me and my children be upset. When will I matter?'" she told Chang.

A rep for Britney has not commented to PEOPLE. Lawyers for Lynne and Jamie Spears did not immediately respond to requests for comment.

In the latest issue of PEOPLE, Jamie Lynn opens up about healing from trauma stemming from pressures she faced in her childhood and her complicated family dynamic.

RELATED VIDEO: Jamie Lynn Spears on Healing After Past Trauma and Mental Health Struggles: I've Found 'Peace'

Since June — when Britney, 40, first spoke out in court against her long-held conservatorship, which was terminated by a judge in November after 13 years — fans of the superstar have attacked Jamie Lynn online, accusing her of not supporting Britney during the conservatorship; she's even gotten death threats. Last week, Britney unfollowed Jamie Lynn on Instagram. (Jamie Lynn still follows her sister.)

"I've only ever tried to be helpful, so any notion that says the contrary is just completely ridiculous," Jamie Lynn tells PEOPLE of her strained relationship with her sister, who has repeatedly criticized her family on social media over the last six months.

For Jamie Lynn, "it was really important for me to separate myself from my family to focus on the family I've created, which is me, my husband and my daughters. Setting those boundaries in place was really important," says Jamie Lynn, who is raising her daughters with husband Jamie Watson, 40. "I am not my family. I am my own person. I fought very hard and worked very hard to establish myself and to build the life that I have today."