To unlock Family Heirloom achievement you need to complete the prologue, this only takes a few minutes.
You start the level submerged in water. Swim forward until you see a chair which swivels to reveal a music box.
You will awake in the cabin of a ship and must collect some items in order to leave.
Pick the gloves up from the center of the desk.
Pick the journal up from the left of the desk. Press Y to open it and see a picture of a house with the number "506".
Interact with the box on the right of the desk and input "506" as the combination. (interact with the rectangular switch to finalise the input).
Pick up the blade in the center of the box and the paper to the right.
Interact with the photograph in the left of the box. Spin it over to reveal a brass key.
Turn around and interact with the door handle to leave the room and pop the achievement.