Are twins hereditary?

Yes and no. Some multiple births occur naturally at approximately the rate of: twins: 1/89 (about 1.1%) of singleton births. North american dizygotic twinning occurs about 1:83 conceptions, and triplets about 1: 8000 conceptions. In the us, there were over 136, 000 multiple human births in 2003. A traditional approximation of the incidence of twins is 1:80. The incidence increased lately due to ivf & iui.
Yes. Usually fraternal or non-identical twins can be hereditary. More often in this day and age twins and higher order birth are related to fertility medications and ivf.