After you've completed all available missions in the game, you'll be given the option to start the mission Family, the final one in the game. This will have you driving all the way back to where the game started. You'll meet Maa Maa here, who will give you the options Proud of Guu Ma and Duck Confit. Go the restaurant and convince your grandma to go eat noodles instead of French food, and then stop at the apartment. You'll get Double Happiness here, then go to the kitchen and put the duck in the oven.
Go the the Reception area to use the Duck Confit option, then take Maa Maa down for dinner and the Duck Confit achievement. Talk some more until you can use Proud of Guu Maa as a choice, this will open the tree to let you say Double Happiness. This will finally unlock the Family achievement which will start rolling the credits and end the game.