Nick Carter and his sister Angel are honoring their late brother Aaron by paying it forward ... raising money for a children's mental health charity.
Nick's rep tells TMZ ... Nick and Angel have started a fund where fans looking to pay tribute to Aaron can donate, in lieu of flowers, and the money will go to a children's mental health organization.
We're told the fund benefits a group called On Our Sleeves: The Movement for Children's Mental Health, and it's a cause Nick's championed for years ... even serving as an ambassador.
On Our Sleeve's mission is to provide every community in America with access to free, evidence-informed educational resources, to break stigmas about child mental health as well as educating families and advocates.
We're told donations will fund educational content, lesson plans for schools and activities for schools and organizations.
Nick's camp says ... "By honoring Aaron, you help Nick, Angel, and their family's commitment of helping others."
TMZ broke the story ... Aaron was found dead earlier this month in his bathtub. The cause of death has not been determined pending toxicology results.