8:26 PM PT -- Sources close to Jamie Lynn tell us she has not yet received the letter.
Well, forget reconciliation ... Britney Spears has threatened to sue her sister, Jamie Lynn, if she dares lie about her while hawking her book.
Britney's lawyer, Mat Rosengart, fired off a cease and desist letter -- obtained by TMZ -- to Jamie Lynn Monday. Rosengart was blunt, demanding, "you cease and desist from referencing Britney derogatorily during your promotional campaign. If you fail to do so or defame her, Britney will be forced to consider and take all appropriate legal action."
The letter, first obtained by Page Six is biting ... Rosengart writes, "We write with hesitation because the last thing Britney wants is to bring more attention to your ill-timed book and its misleading or outrageous claims about her."
Rosengart also says, "Britney has not read and does not intend to read your book, she and millions of her fans were shocked to see how you have exploited her for monetary gain. She will not tolerate it, nor should she."
And then he dumps hard on Jamie Spears ... "You of all people know the abuse and wrongdoing Britney had to endure during the conservatorship, after initially growing up with a 'ruinous,' alcoholic father." He points out Jamie echoed the same sentiment in her book.
And then he goes after Jamie Lynn ... "Britney was the family's breadwinner and she also otherwise supported you. Publicly airing false or fantastical grievances is wrong, especially when designed to sell books. It is also potentially unlawful and defamatory."