Jamie Lynn is reacting to Britney's reaction to her book, "Things I Should Have Said." Jamie Lynn says in the memoir ... Britney exhibited paranoid, erratic behavior over the years, and even talked about an incident where Britney locked the two of them in a room with a knife, saying she was scared.

Britney lashed out at her younger sister, posting on social media Jamie Lynn was "never around me much 15 years ago [when the conservatorship began], so why are they even talking about that unless she wants to sell a book at my expense??? REALLY???"

Britney wasn't done ... "She never had to work for anything. Everything was always given to her!!!!" She says her family "ruined my dreams 100 billion percent and try to make me look like the crazy one."

Jamie Lynn wasn't shy about responding ... "It's become exhausting when conversations, and texts we have in private don't match what you post on social media. I know you're going through a lot and I never want to diminish that, but I also can't diminish myself."

Jamie Lynn went on to say she and her family have been getting death threats, and she blames Britney, saying her "vague and accusatory posts" have triggered her fans, adding Britney could "tell the truth, and put an end to all of it in one second if she wanted to."

And, then she leaned in even harder ... "I hate to burst my sister's bubble, but my book is not about her. I can't help that I was born a Spears too, and that some of my experiences involve my sister. I've worked hard since before I was even a teenager, and I've built my career in spite of just being someone's little sister."