1. Probably the easiest, most fail-safe way to conceive sim twins/triplets is if you purchase fertility treatments for the mum and dad. To do this, you need at least 10 000 happiness points (per sim). It sounds like a lot, but it really isn’t. In my experience, if one parent has it you’ll get twins, and if both do you will have triplets.
2. If your sim gets pregnant without the treatment, you can increase the likelihood of having twins by having the mum do childish activities. Watch KidZone on TV, read pregnancy books, listen to kids’ ptrrograms on the radio etc.
3. You can also control the gender. If you want boys, have the mother eat three apples consecutively (in the first 2 days of the pregnancy). If you want girls, have the mother eat three watermelons.
Have fun!! xD