Ariana's big bro took to X Thursday to fire at fans spreading rumors that cops found human remains in his sister's starter house years ago ... cackling over the crazy claims.

If you haven't seen the original TikTok, check it out ... it's a social media user making the claims without providing any sort of evidence to back up the point -- but, some people took it as fact and started pushing the rumor anyway.

FG says the fans who fanned the cannibal flames have hit a more creative -- and lower level -- than they've ever gone ... reaching deeper depths with each passing day according to the Broadway star.

Of course, Frankie wanted to hype up his family too ... putting the focus back on her success, by adding, "Listen, I know my sister’s been eating the girls up for years but this a bit extreme!" before pointing out how she's a vegan.

Worth noting, Ariana found herself in a cannibalism controversy last month ... when she said during a podcast interview with Penn Badgley she'd want to have a sit-down with infamous cannibal Jeffrey Dahmer.

We broke the story ... the family of Jeffrey Dahmer victim, Tony Hughes, blasted the pop star -- saying she should apologize and adding it makes her seem "sick in her mind."

As for actually eating human flesh, seems a step too far ... and, Frankie's making it clear Ariana's only eating up people on the mic.