You see, her cousin Mia had just been born (a mere five minutes!) before the video was made. When her mother breaks the news that Mia has to stay at the hospital for a few more days, Sierra puts on the waterworks.
Be sure to watch all 69 seconds of the clip, especially the part where Sierra is told about having a birthday party next year.
Happy Friday, and hug a birthday girl (or toddler) this weekend.