Story related but missable.
Make sure to use a save where Bethany or Carver is available to have in your party. Then make sure you keep them in your party from beginning to end. Changing them out at an point will null the achievement.
If you do not have a save file where Bethany or Carver have survived the deep roads expedition, you must start a new game and make it to Kirkwall. When you reach the Deep Roads, either leave them at home, or take them and Anders with you on the expedition. This will allow you to have your sibling join the grey wardens and save them from the corruption.