This is so super adorable!
We sometimes forget that little kids don't know even some simple facts of life. Like that as time goes on, people age and eventually people die.
She's crying because her baby brother is sooooo cute as a little baby and she doesn't want him to grow up!! Also, she doesn't want to die when she's 100, which is understandable.
We feel bad she's so sad, but it's the cute kind of sad — especially if you like seeing someone having their first taste of an existential crisis.
Check out the video (below) and prepare to awwww your face off!!