Whitney's nephew went nuclear on Bobbi Kristina's husband

Whitney Houston's son-in-law got his ass beaten by one of Whitney's nephews at a formal family gathering  and it's all because of a long-simmering family feud .

We're told Whitney's nephew, Gary Michael Houston, went nuclear on Bobbi Kristina's husband, Nick Gordon slamming him to the floor and pummelling his face accusing him of talking crap about the family.

According to TMZ,  it all went down at a Sweet 16 party for Whitney's niece, Rayah. We're told it was a formal affair everyone in tuxes and long gowns but Nick showed up in a hoodie and knit cap and "lookin' like a thug."

Gary felt Gordon had insulted the family one too many times  as soon as the party was over, he delivered the beat down. We're told it was a lopsided fight  Nick didn't even get in a single punch.

The two are now sluggin' it out on Twitter. Gordon posted, "F*** all of em they've tried to fight me (Gary H) jealous ass n***a."  

In response, Gary tweeted, "He still kind a sore from that last trip I sent him on smh I tried to tell him to buckle up lol."