Instructions
Start a new game and enter "Create-a-Family" mode to make a new Sim family.
Click the 'Add Sim' button to start working on the first twin boy. Type a name for the Sim and set his gender to male and his age to teenager.
Design your Sim however you like by using the tools in Create-a-Family mode. Select the Sim's skin color, weight, hairstyle, facial structure, body type and wardrobe.
Write your first twin's settings on a pad of paper so you can apply the same settings to the second Sim.
Save your Sim.
Click the 'Add a Sim' button to start work on the twin brother. Refer to your notes and choose the same settings for the second Sim to create an identical twin. You can add any variations you like, such as giving the twin a different name and outfit.
Save the Sim and go back to the family screen. Click one Sim's icon and drag it to the other Sim's icon. Select 'Brothers' to make the Sims twin brothers.