This is missable. You must read the minds of the three Gremory siblings throughout the game in order to obtain it.
1) The first you meet is George in Broken Valley, you can find him close to the southmost teleport in this area
(** It is essential that you do this mind read before you progress past Maxos Temple as this area is no longer available after doing so **)
2) Catherine is in the Sentinel Island, at the huge whale corpse.
(** This is also missable, as you must do this before you gain the dragon tower and the map becomes engulfed in poisonous mist **)
3) The easiest, and last of the siblings is in the imp cave in Orobas Fjords, fly with your dragon south from the entrance to Stone's fortress. There is a wooden hatch opposite the middle of the island. He is held as a prisoner, you should have little trouble in finding him.
Once you have read the minds of all three you will unlock the achievement.