"You've stooped to a whole new level of LOW," Britney Spears wrote of younger sister Jamie Lynn recounting an incident when the pop star allegedly locked them in her room
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Britney Spears is telling her side of the story.

In a Twitter post Friday afternoon, the pop star, 40, accused her younger sister Jamie Lynn Spears of spreading "crazy lies" after Jamie Lynn, 30, alleged during her recent interview with Nightline that Britney once pulled out a knife and locked the two of them in a room.

"Jamie Lynn... congrats babe! You've stooped to a whole new level of LOW... I've never been around you ever with a knife or would I ever even think to do such!!!" Britney wrote. "The only knife I ever saw you with at home was cutting the biggest pieces of squash I ever saw in my life and it was way too big for me to cut... So please please stop with these crazy lies for the Hollywood books!!!

"NOW and only NOW do I know only a scum person would make up such things about someone.... I'm actually very confused about you making that up because it's honestly not like you at all!!!! Around the kids???? Jamie Lynn, seriously??? Come on!!! Congrats on introducing your older sister the concept of getting LOW... LOWER... LOWEST... because you win on that one, babe!!!!"

Jamie Lynn previously told Nightline's Juju Chang that Britney has often exhibited "erratic, paranoid and spiraling" behavior, recounting a time when her older sister once became "scared," took out a knife and locked the two of them in a room.

"It's important to remember that I was a kid in that moment," Jamie Lynn said. "I was scared. That was an experience I had, but I also was fearful to really say anything because I didn't want to upset anybody — but also was so upset that she didn't feel safe."

Britney has since clapped back at the Zoey 101 star, accusing her Thursday evening of trying to "sell a book at my expense," as Jamie Lynn promotes her upcoming memoir Things I Should Have Said, out Jan. 18.

Jamie Lynn shared a message on Instagram early Friday insisting that she "is always here" for her older sister while explaining the accusations Britney makes on social media "don't match" what is said between the sisters privately. She claimed Britney's posts lead to "death threats" against her family. 

"Brit, I am always here for you, you know behind the scenes I have always been here. It's become exhausting when conversations, and texts we have in private don't match what you post on social media," Jamie Lynn wrote. 

Jamie Lynn has repeatedly said she supports Britney, while her older sister has denied her claims and previously accused her family of conservatorship abuse. Her father, Jamie; her mother, Lynne; and Jamie Lynn have all denied wrongdoing. 

Jamie Lynn Spears
Britney and Jamie Lynn Spears in 2017
| Credit: Image Group LA/getty

In the latest issue of PEOPLE, Jamie Lynn opens up about her complicated family dynamic and the pressures of her childhood in the spotlight, as well as her efforts to heal from the resulting trauma.

"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," Jamie Lynn said of daughters Maddie Briann, 13, and Ivey Joan, 3, as well as 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."

RELATED VIDEO: Britney Spears Calls Out Sister Jamie Lynn amid Book Release: 'She Never Had to Work for Anything'

Since June — when Britney 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.