A jury at Manchester Crown Court took four hours to find the 15-year- olds guilty of killing Lily Lilley then dumping her body in a wheelie bin and abandoning it in a canal.
The hearing was adjourned until today, when the judge will consider lifting a ban on identifying the girls.
The girls wept uncontrollably when the verdict was given. After a brief adjournment for them to compose themselves, Mr Justice Sachs, passing sentence, said: "You have both been convicted of an act of unbelievable cruelty. Two young girls for totally pathetic reasons destroyed a frail old lady in order to get her house.
"What you did was unspeakably wicked.
"You have not shown the slightest trace of remorse or regret. You are both hard young women who have committed a terrible crime which almost defies belief by right-thinking people.
"Your only mitigation are your ages. You are both 15."
During the six-week trial, the court heard that the girls befriended Mrs Lilley then murdered her at her home in Failsworth, Greater Manchester. They wound a bandage around her face so tightly she choked on her false teeth then slashed her legs with a blunt knife and forced her eight- stone body into a wheelie bin. They pushed the bin through the streets and dumped it into the Rochdale canal.
The girls then made Mrs Lilley's house their own, using the telephone 258 times. They wrote their names on the walls and carved them into the furniture.
Detective Superintendent Roy Rainford, of Greater Manchester police, said the jury had delivered a "true and just verdict". "I find it inconceivable that any person, especially two young girls, could commit such a wicked crime."Reuse content