This article falsely says that a twin study assumes twins are treated equally. Actually, twin studies don't assume that two twins are treated equally to each other. Rather, the studies assume that fraternal twins are treated no more similarly to each other than "identical" or monozygotic twins are. This is called the "equal environment assumption" and is generally wrong.
Source: "Twin Studies in Psychiatry and Psychology: Science or Pseudoscience?" by Jay Joseph, Psychiatric Quarterly Spring 2002. Full text available online.