on sims 3 have the fertility treatment on both the sims and then while they are showing make them listen to kids music or you will end up with twins if you do it wrong
on sims 2 you will have to get a hack
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What you do is when your sim is pregnant and having the baby stay at home and type Ctrl+Shift+c and type in twins or triplets if you want triplets and there you have it you have twins!!!Your Welcomelook me up on youtube as dolia3654
To get twins, you can use the cheat "forcetwins" in the cheat console after your sim is pregnant.
i dont know how to get triplets, but if you wait until right after you sim gets pregnant, click on her, press ctrl+shift then c and type in force twins. works everytime!
Have 'woohoo' whilst listening to Kids music and have at least one 'parent' Sim with the perk 'Fertility Treatment'.