Preparation - Reunion
Upon reaching the trailer park, you can find your family at their trailer. After a brief cutscene, Leo will be asked to talk to Alex, but your focus should be on Vincent. Have Vincent talk to Leo's wife who is working on a motorcycle and offer to help. She'll ask you to grab something for her from the shed. Return this item to her and follow the button prompts to start the bike to unlock the achievement. Make sure you do this before having Leo visit Alex in the treehouse.
This achievement can be seen in the video guide at 11:15:NOTE:
Achievements are only available to owners of the game. Players using the "Free Trial" friend pass will NOT unlock achievements.