The nephew, also named John Mara, is scheduled to appear Tuesday in Bridgeport Superior Court. His attorney, William Dow III, said Mara will plead not guilty to first-degree assault and breach of peace charges.
Police say a witness identified Mara as the person who committed the assault. Mara, who lives in Mount Kisko, N.Y., was arrested in February.
An arrest affidavit says the victim, Philip Blackman, suffered life-threatening injuries that required immediate surgery.
First-degree assault carries a sentence of one to 20 years in prison.