Create a household before you leave the Create a Sim screen.
The term for a group of Sims living in a single home is a household. A household can consist of a single Sim, or two or more Sims who are roommates, siblings, or a complete family with adults and children. A household is limited to eight Sims, but you can have any combinations of ages and genders that you want (as long as you include at least one Sim that is Young Adult or older). While the last chapter focused on the intricacies of Create a Sim, this section touches on the creation of a household of Sims.
Play around with the various types of Sims you can create with genetics. In addition to creating children, you can also create parents for your Sims. They always start older than your Sims and you can make them even older as long as it’s within the allowed range..
Once you have created your first Sim, you can either have that Sim live alone or add other Sims to the household. After creating an additional Sim, click on the Edit Relationship button (just to the right of the menu for setting your Sim’s voice). If the two Sims are the same age, you can choose to have the Sims be roommates, siblings, or spouses. If the Sims are of different ages, they can be roommates, children, or parents. This allows you to create various types of relationships within the household. These previously created relationships let you start the game with romantic relations if your Sims are married and also opportunities for socialization right from the get go.
Using genetics, you can create a twin for your Sim. It can either be a brother or a sister and will have many of the same physical features as your Sim.
As mentioned in the previous chapter, you can play with genetics to create your household. While you usually use this to create a child for two Sims, or a parent, it is fun to create genetically linked siblings. Select the Sim for which you want to create a brother or sister, then use the Play with Genetics ability when you create a new Sim. In addition to making a brother or sister, you can also make that sibling a twin.