Heartbreaking video shows Texas boy sing to baby sister in heaven

SAN ANTONIO, Texas -- Siblings have a unique and special bond. Sometimes that bond is forced to endure heartbreaking loss.

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.

On what would have been little Ava's first birthday, Alex grabbed his guitar and sang to his baby sister in heaven.

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.

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.