Well somewhere on the net it said that if the parent is a twin the chances are higher and I think with each successive generation the probability increases if that parent is one of a twin.
but ohterwise i think its like a 10% chance to have twins at random, however if you have OFB you can use the cheat 'forcetwins'
(Type: Forcetwins while she's in labor. Obviously, type it before it says there's a __________________________ on the way. Please wait while the yada yada yada... This GUARENTEES twins.) or get the sim to eat cheescake.
There is supposed to be a hack somewhere were you can change the probability of your sims having twins (even triplets and quads which are not possible with the normal game, )
You can find it
Here I know it says Triplets and Quads, but Im sure the Twins and Single pregnancy is included.
PS Sorry for the lengthy post hehe