The Backstreet Boys singer had earlier accused Aaron of threatening to kill his wife Lauren Kitt and their unborn child, and said he was showing "increasingly alarming" behaviour. Now, Angel has spoken out in a statement on social media, reports metro.co.uk.
"Today I filed a protective order with my brother Nick in Los Angeles. Following a recent relapse, Aaron confessed to me he had violent thoughts towards babies and our family members," she wrote.
"This, combined with his recent access to assault rifles, has made us afraid for our safety."
The 31-year-old added: "It is clear to those of us that love him that Aaron is suffering from a mental health crisis. Unfortunately this has led to his fabricating stories in order to divert attention away from the truth. We have been working closely with law enforcement who share our concern surrounding his current mental state.
"We want our brother Aaron, whom we love very much, to get the help he so desperately needs but we have a baby girl and based on the alarming events that Aaron has threatened to carry out, we are compelled to take action to protect our family."
Meanwhile, Nick has shared a photo of himself resting his head on his wife Lauren's baby bump, writing: "We just want to thank everyone for all the overwhelming love and support being sent our way. We are happily waiting for this little miracle to arrive. #love #peace #rainbowbaby #family."
Following Nick's initial statement about the restraining order, Aaron began to re-share articles about a rape allegation made against his brother by a singer.
Nick had denied the allegations - filed last February in regards to an alleged 2003 incident. In September 2018, it was announced that the LA County District Attorney's Office would not be pressing charges.
( With inputs from IANS )