Nick Carter is putting his feelings about his late brother Aaron Carter on wax -- as they used to say -- writing and recording a song where he sings about their rollercoaster relationship.
Music industry sources tell TMZ ... Nick got to work on the song shortly after Aaron died in November and as you'd expect -- considering the brothers' history -- it's a very emotional track.
Despite their complicated relationship, we're told the song expresses Nick's unconditional love for Aaron and his heartbreak over his brother seemingly never finding peace in his life.
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Nick's been teasing a new project on social media, and we're told he also filmed a music video for the song ... which includes never before seen footage of Nick and Aaron together from when they were younger.
Remember, the Carter bros had tons of ups and downs over the years ... with one of their lowest points coming in 2019 when Nick got a restraining order after claiming his younger brother threatened to kill Nick's pregnant wife and unborn child.