Mirai was apparently inspired by the behavior of director Mamoru Hosoda's own young son toward his even younger sister. And so we have the story of spoiled four year old boy Kun (voiced by Jaden Waldman in the English version) and his frustrations when all his parents' attention goes to their newborn daughter Mirai. The movie becomes something of a replay of "Scrooge," with Kun meeting various members of his family (notably Mirai - which means "future" - from the future ...) and beginning to see the why and how of the history of his family.
It's a charming movie, ranging from broad comedy through gorgeous images to some interesting commentary on parenting (which are brief enough to not distract the kids who won't really get it) and finally landing on Kun growing up a bit. Charming and heart-felt, but if you want to watch Hosoda at his best, watch "Wolf Children" (I didn't like it, but a lot of people did and I understand why), or - best of Hosoda's previous movies - the outstanding "The Boy and the Beast."