Here is the full list of all 12 Brothers: A Tale of Two Sons achievements worth 400 gamerscore.
You practised singing.
You helped the turtles to the sea.
You found a secret.
You sounded a giant horn.
Every family's got one.
You made the bunnies play nice.
You threw someone's ball down a well… Shame on you.
You took a break from adventuring.
You re-united the two love birds.
You made the inventor dance.
You made someone feel better.
Make a wish.