J.J. Watt's younger bro, Derek Watt, isn't just praising his brother for raising $27 MILLION for Hurricane Harvey victims ... he's getting involved, too! 

Derek -- a 2nd year fullback for the Chargers -- was loading up trucks at the "Chargers Food & Supply Drive to Aid Hurricane Harvey Relief Efforts" in L.A. on Wednesday ... along with several teammates. 

Watt says he's been texting with his family about J.J.'s fundraising -- and thinking of other ways they can help out. 

You gotta watch the video ... Derek says he's "extremely proud" and vows to help push the magic number to over $30 MILLION. 

Props to everyone involved.