Out of the blue, five-year old Sadie Miller burst into tears thinking about mortality. She didn't want her baby brother to grow up, nor did she want to die when she turned 100 years old.

And dad Ryan got it all on camera.

After the heartwarming video was shot, Ryan's brother Alex uploaded the video on YouTube.

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The viral video shows the adorable girl from Phoenix, Arizona sobbing while her three-month-old brother smiles in the background.

Wearing a pretty Princess Sofia dress, Sadie cries and embraces her younger brother, Carson. She lovingly kisses him on the head and says "I don't want him to grow up!"

Subtitles guide viewers along Sadie's sad soliloquy, which she manages to say through sniffles and tears.

Sadie ends the intense monologue with a phrase she might recall when both children get older and start fighting over who gets to drive the car: "OH MY GOSH! I want him to stay this little." To which cheeky little Carson responds with giggling.

Within 24 hours, the popular YouTube video garnered over 48,000 views and viewership is still growing.

One YouTube user hilariously commented, "Wait till you are 28 and the bills start stacking up. Even if you have a good job it is never enough. You will wish you will die on the ride to work the next day for the rest of your life. haha"

Dad Ryan said that Sadie has cried about getting older before, but this was the first time she got emotional about her brother getting older.

"The funny thing is that this was totally out of the blue - we have no idea what prompted it," Ryan said.

Sadie is said to be extremely devoted to little brother Carson and loves reading him to sleep. Theirs might be one of the cutest sibling relationships yet - at least among all those caught on video.

TagsLittle girl, crying, baby brother growing up, Video