"Jamie Lynn... congrats babe! You've stooped to a whole new level of LOW," Britney began of her sister, 30. "... I've never been around you ever with a knife or would I ever even think to do such!!"
"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," Britney added. "... So please stop with these crazy lies for the Hollywood books!!!"
Britney's comment is in reference to a story from Jamie Lynn's upcoming memoir. In Jamie Lynn's interview with ABC News, she discussed the alleged incident, in which Britney, with a knife, locked her and Jamie Lynn in a room and said she was scared, according to the younger Spears.
"NOW and only NOW I do know only a scum person would make up such things about someone," Britney wrote. "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 [to] the concept of getting LOW... LOWER... LOWEST... because you win one that one, babe!!!!" Britney concluded.
In reference to the alleged incident with a knife and others described throughout the memoir, Jamie Lynn told ET, "I think there was a lot of things that happened in my childhood that were very scary and unsafe."
"I just think that it’s important that I was able to share how I felt in those moments, no matter who else is involved in them," Jamie Lynn said. "I just want to share my experiences. It's scary, but, at the same time, it's important."
Friday's post isn't the first time that Britney has spoken out against Jamie Lynn's memoir. The day prior, Britney wrote on Twitter that she watched Jamie Lynn's interview when she was suffering from a 104-degree fever.
"It was actually kind of nice having a fever so high cause I had to surrender to not caring. I just couldn't give a f**k but my head hurt so bad," she wrote in part. "The 2 things that did bother me that my sister said was how my behavior was out of control. She was never around me much 15 years ago at that time... so why are they even talking about that unless she wants to sell a book at my expense?"
Shortly thereafter, Jamie Lynn responded with a statement of her own, and accused her sister of misrepresenting their relationship.
"Brit, I am always here, 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 in part. "I know you're going through a lot and I never want to diminish that, but I also can't diminish myself."
"There are no sides, and I don't want drama, but I'm speaking my truth to heal my traumas, so I can close this chapter and move forward, and I wish my sister could do the same," she added. "No matter what comes, I will always love my big sister, and be here for her. It's time to put an end to the unhealthy chaos that has controlled my life for so long."
Prior to the back-and-forth social media posts, Jamie Lynn told ET that her book "isn’t about me trying to clear up anything," but rather about "using my voice and showing my daughters that your voice is important." Jamie Lynn is mom to Maddie, 13, and Ivey, 3.
"You should always feel like you can control your narrative and I want to make sure I do that and I hope that others can do the same," she told ET. "And also, to have understanding for other people's stories. We don't all know everyone's full story and what they've been through, what they're going through. If we can create a safer space for other people to share what their voice is, then I think that maybe we'll have a lot more healing in this world and a lot less judgment."