Pablo Escobar's brother just put Netflix on alert -- cut us in on "Narcos" ... or else.

Roberto Escobar -- the accountant in Pablo's empire -- fired off a letter to Netflix in which he demands the media company put the brakes on season 2, unless the company gets a sign off from him.

He claims season 1 was riddled with inaccuracies and lies and says his brother would not like the portrayal one bit.

Roberto calls his letter -- obtained by TMZ -- "a friendly request for cooperation." He wants a cut of the profits.

The letter itself is interesting for several reasons. Our sources say it was written by Roberto at an undisclosed location in Colombia. And it's stamped with the Escobar family seal.

Roberto claims undying loyalty to Pablo, saying, "I was Pablo's closest ally, managing his accounting and he is my brother for life."