Logan Paul's a "fake fighter" who has no business being in the ring with Floyd Mayweather ... so says his own brother, Jake, who tells TMZ Sports the dude is "f***ked" against the 50-0 boxer.
Of course, the YouTube superstar is set to face off against "The Best Ever" in a boxing exhibition on Feb. 20 ... a fight that has been met with mixed reactions.
Jake -- who's fresh off an insane KO of ex-NBA star Nate Robinson -- thinks Logan is pretty much doomed against Floyd ... and believes the fight is doing more harm than good for boxing.
"My brother's f***ed," Jake tells us. "It's bad for the sport. I think it's just for clout."
Paul is echoing what Canelo Alvarez recently told us ... saying he thought it was a "lack of respect" for pros to go up against YouTubers.
Jake goes on to say he's the "real fighter" in the family ... pointing out he's got a 3-0 record ... while Logan still hasn't racked up a win.
"I wish him the best of luck. I just like -- don't get in there with a guy whos' never even been knocked out. He's been knocked down once and it was by accident."