This is missable. After completing the main story quest "A New Home", you must talk to your mother in Gamlen’s House and then speak with your brother or sister to begin the companion quest, "Birthright". Travel to your old estate in Darktown and fight your way to the Vault. Once inside, loot the chests and you will find your grandfather's will to unlock the achievement. See the descriptions for "Gift Giver" and "A Friend in Need" for how to unlock them all during this quest!
Note: You must complete this quest before leaving for the Deep Roads or you will have to do it on another playthrough.