Here is the full list of all 12 Brothers - A Tale of Two Sons achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
You found a secret.
You re-united the two love birds.
You made the inventor dance.
You made someone feel better.
Make a wish.
You took a break from adventuring.
Every family's got one.
You made the bunnies play nice.
You threw someone's ball down a well… Shame on you.
You practised singing.
You helped the turtles to the sea.
You sounded a giant horn.