Mirai

 One day, Kun’s mother brings home a new family member. For the four-year-old boy, joy quickly turns to disappointment when the little sister replaces her brother as the center of attention. The boy can’t bring himself to accept her as his sister. But things turn magical when, with a mysterious garden as his gateway, the boy encounters his mother as a little girl and his great-grandfather as a young man, and has a series of surprising adventures with his older sister from the future. These experiences change how he sees the world and help him grow into the big brother he was meant to be.

(Source: GKIDS)