Been having the same issue with the sims in my neighborhood, since the Holiday Patch each and every time any of my sims gives birth it is ALWAYS twins (a variety of girl-girl, boy-girl, and boy-boy twins). It has come to a point where my sims are having so many multiples it is not realistic towards my neighborhood's story progression. Sooo.... what I have been doing as of lately is if a sim has a set of twins I will make one of the twins age up and then I go to CAS mode for that particular family and delete one of the twins.