You have to watch this video of Jessica Biel's little brother Justin Biel...trashing her BF Justin Timberlake. Apparently he's upset with the way JT handles his company "Tequila 901"

Jessica Biel's little brother is slamming Justin Timberlake's 901 Tequila company -- claiming the brand is a massive failure and JT is to blame.

Justin Biel -- who we're told worked for the 901 brand until a few weeks ago -- just posted a couple of videos to his Facebook page ... in which he claims Timberlake sabotaged the company by refusing to promote his own liquor.   

Biel rants, "I've been waiting for [Timberlake] to do something with this f**king brand -- make a song about it, put it in a god damn video, hold it up, take some pictures, I don't care -- just do something because we're dying on the motherf**king vine here!"

Biel -- seemingly bitter with the brand -- sarcastically launches into a series of new "angles" he thinks could help sales ... claiming among other things:

-- "901 Tequila ... mixes well with a bump of cocaine"
-- "901 Tequila ... Steve Jobs used to drink it ... oh man that's too soon."
-- "901 Tequila ... I only s**t my pants once on it."
-- "901 Tequila ... so god damn exclusive our own CEO doesn't even drink it."

Biel continues, "Sales dropped drastically from last year ... way to go sales team!  Our distribution partners still couldn't give two f**ks about the brand, repping Patron all the way, and giving us the ol' fist up the ass."

Sources connected to Biel and Timberlake tell us Jessica's brother left the company on good terms ... even though it doesn't appear that way in the videos.

A rep for 901 tells TMZ ... "Justin Biel’s video was intended to be one in a series of videos created by 901 Tequila employees (including the brand owner) as part of a larger viral marketing campaign slated for the end of December. It was released prematurely without our knowledge and prior to the final edit.”