Three months before she was stabbed to death in her Double Bay apartment, Sydney restaurateur Rita Caleo gave a sealed envelope to her solicitor.

The envelope read: "To be opened only if my death is unnatural."

Stabbed to death in her home: Rita Caleo.

Inside the envelope, which Mrs Caleo's solicitor opened in the presence of detectives following her murder in 1990, were the names of two people she accused of organising the gangland-style execution of her brother, a wealthy doctor, at his Woollahra mansion the previous year.

If she died an unnatural death, the same people would be responsible, Mrs Caleo wrote in the letter.