Instructions
Twin Cheat Code
Get the female character pregnant using the "Woohoo" feature of the game. To determine if the character is pregnant, listen for "Rock a Bye Baby" after the "Woohoo" session or wait to see if the character begins running to the bathroom with morning sickness.
Enter the code CTRL+SHIFT+C (or CTRL+SHIFT+WindowsKey+C for PC using Microsoft Vista) to bring up the cheat console.
Enter "testingcheatsenabled" and "true" to enable to cheat.
Select the pregnant character and reenter the code CTRL+SHIFT+C or CTRL+SHIFT+WindowsKey+C.
Type in "Force Twins." The Sim will now have twins.
Increase Chance of Twins
Obtain the "Fertility Treatment" for both the male and female parents. The treatment can be obtained using reward points or can be purchased at the hospital. Having the treatment on both parents raises the odds of having a multiple birth, including triplets.
Make the characters "Woohoo" and watch for pregnancy signs in the female character.
Make the female character read pregnancy books, watch Kid's Zone TV and listen to children's music during the pregnancy to increase the odds of having twins or triplets.