UFC champ
is being CALLED OUT by his own brother -- NFL star
Chandler Jones
-- who tells
TMZ Sports
he's ready and willing to step in the octagon with JJ 'cause he's 100% confident he could beat his ass. 

"Me and my brother used to fight all the time when we were young ... and I'm pretty sure I can take him down now. I am way bigger than he is," Chandler tells us.

For the record, Jon is the reigning UFC light heavyweight champ. Chandler just won the Super Bowl.

By the way, Chandler is dead serious ... and says he wants to set up a fight for charity.

Read more: