Angel Conrad filed documents on Friday, September 20, in Los Angeles, according to TMZ.
Nick filed for protection against his brother on Monday, September 16, claiming Aaron had told his sister he harboured "thoughts of killing babies" during a recent FaceTime chat - and he feared for his pregnant wife and unborn child.
The Backstreet Boys star also alleged Aaron told Angel he had been diagnosed with schizophrenia and bipolar disorder and confessed he was afraid his brother will listen to the "voices in his head and cause harm to my family."
His little brother has denied all of Nick's claims.
Nick has also released a statement about his restraining order request. "In light of Aaron's increasingly alarming behavior and his recent confessions that he harbors thoughts and intentions of killing my pregnant wife and unborn child, we were left with no choice but to take every measure possible to protect ourselves and our family," he tweeted.
In response, Aaron said, "Anybody who knows me knows I wouldn't hurt a fly."
The youngest Carter brother has since accused his brother of bearing a grudge because he didn't support him after actress and singer Melissa Schuman accused Nick of raping her. Aaron has also sensationally claimed he was raped by his late sister, Leslie, when he was 10.