Bobbi Kristina -- Bobby's Sister Rails on Pat Houston ... You Want BK to Die!

Bobbi Kristina's aunt is trying to kill her ... so claims Bobby Brown's sister.

Leolah Brown unleashed a ferocious rant ... claiming Pat Houston -- Whitney's sister-in-law and Bobbi's co-guardian -- is so motivated by greed she wants her niece to die.

Leolah claims Pat callously complained, "Why don't he (Bobby) just pull the plug on Bobbi Kriss, because it's costing too much money to keep her alive."

Leolah claims Pat believes she'll inherit Bobbi Kristina's money and she's worried the costs associated with medical care will deplete the fortune.

And Leolah twisted the knife with this ... "Whitney Houston HATED THE VERY GROUND PAT WATSON WALKED ON AND PAT AND THE HOUSTON'S KNOW THAT THIS IS TRUE!!"

Bobbi Kristina has been hospitalized since January 31.