The sister of "Big Brother" contestant Jocasta Odom is tearing cast member Frankie Grande to shreds ... for saying Jocasta "can kill herself."
Frankie's back stab stung hard ... because Jocasta -- a pastor -- just performed a memorial service for his recently deceased grandfather.
Maranda Chelsea, Jocasta's sis, tells TMZ, "It was an ignorant statement and the little respect I had for him just went out the window." And the parting shot, "It shows the lack of maturity he has."
Miranda isn't the only one pissed off -- so are the fans ... @JrAguja1997 tweeted "I don't f**king care if your sister sings good, you're a s**tty person Frankie. Calling Jocasta to kill herself. Argh."
Whatever sympathy Frankie had built up this past week has now quickly gone away ... and it only took 13 seconds of footage to do it.