Federal prosecutor Elliott Jacobson told a judge Wednesday that 53-year-old Narcy Novack and her brother schemed to kill 86-year-old Bernice Novack at her Fort Lauderdale home last year.
Florida authorities previously ruled Bernice Novack's death an accident. No charges have been filed.
Narcy Novack, her brother and two other men are charged in the killing of Ben Novack Jr. last July at a hotel in Ryetown, New York. Her attorney called the prosecutor's newest allegations "purely speculative."
Ben Novack's father founded Miami Beach's famed Fontainebleau Hotel. Narcy Novack has not entered a plea in her husband's death.