The singer, 39, said the legal move came “in light of Aaron’s increasingly alarming behaviour”.
Aaron, 31, must now stay at least 100 feet (30 metres) away from Nick, his family and their Las Vegas home.
In response, Aaron Carter said he was “astounded” by the accusations.
“I do not wish harm to anyone, especially my family,” he went on.
“After careful consideration, my sister Angel and I regret that we were required to seek a restraining order against our brother Aaron,” Nick Carter said on Tuesday.
“In light of Aaron’s increasingly alarming behaviour and his recent confessions that he harbours 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.
“We love our brother and truly hope he gets the proper treatment he needs before any harm comes to himself or anyone else,” Carter continued.
Aaron, who rose to fame in the late 1990s as a child singer, acknowledged the restraining order on social media but denied the allegations of threatening behaviour.
“Take care Nick Carter. We’re done for life,” said the singer, who has a history of drug convictions and mental health issues.
On a recent episode of US TV show The Doctors, Aaron said he suffered from multiple personality disorder, schizophrenia, acute anxiety and manic depression.
He later said he did not have any mental health issues and claimed the show had been edited to make it look as if he did, which the show’s producers denied.
Nick Carter, whose band had hits with such songs as Everybody and I Want It That Way, announced in May that he and wife Lauren were expecting their second child.
Last year it was decided the older Carter sibling would not be charged over a sexual assault accusation from 2003.