The nephew, also named John Mara, was charged with first-degree assault and second-degree breach of peace and surrendered to police in Fairfield on Friday.
He was released after posting $150,000 bail, police said.
The unidentified victim suffered "life-threatening injuries that required immediate surgery," police said in a statement.
Calls to Mara's home and lawyer were not immediately returned.
Mara is scheduled to be arraigned in Bridgeport Superior Court on March 8.
With The Associated Press