Ariana Grande's Grandpa Died -- Not Telling Bro Until 'Big Brother' Is Over

"Big Brother" is now keeping a deep secret -- something that could derail the show if revealed too soon -- and it involves the death of one of Ariana Grande's relatives.

Ariana's grandfather died Tuesday. He's also the grandpa of Ariana's brother, Frankie Grande, who's a contestant on "Big Brother."

It seems just before he died the grandfather knew Frankie might bail from the show if he found out, so one of grandpa's last wishes was to keep his death a secret from Frankie.

Ariana was chronicling her grandpa's illness ... recently posting pictures with him in the hospital. But Frankie never saw them per the rules of the show.

Ariana made her grandfather's wishes clear, tweeting, "asked grandps last week if he wanted us to pull frankie off bb & he said "no way! he's gotta play the game even tho i don't understand it"

It seems from the "Big Bro" live feeds ... Frankie is still in the dark over his grandpa's death.

7:00 PM PT -- Producers just told Frankie about the death.