Alex is just 4 years old and lives in San Antonio. In May, his 4-month-old sister Ava Lynn died after being diagnosed with hydronephrosis.
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On what would have been little Ava's first birthday, Alex grabbed his guitar and sang to his baby sister in heaven.
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His mother captured the video of the little boy singing "Remember Me" from the movie "Coco."
You can overhear some of the heartbreaking lyrics, which include "though I have to travel far, remember me. For even if I'm far away, I hold you in my heart."
The video was posted on Twitter and has been viewed thousands of times.
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The family says Alex didn't even know he was being filmed and that he just wanted to sing for his sister on her birthday.
Hydronephrosis is a condition that affects the kidneys.