Yes, many mothers continue to breastfeed through a subsequent pregnancy. It is considered safe to do so if you have no risk factors for preterm labor or birth. (Twin pregnancy is a risk factor, but a prior twin/multiple pregnancy is not a risk factor in subsequent pregnancies — unless you are carrying multiples again.) Because you are nourishing yourself, two young infants and your unborn baby, be extra sure to eat a nutritious diet during your pregnancy.
Milk production tends to drop at about three months into pregnancy, when the placenta takes on a greater role in pregnancy hormone production. Some babies wean themselves from the breast at this time. Others, especially younger babies such as yours, will need more supplementary nutrients. If either of the babies lets you know that milk production has changed you can start by feeding them any expressed milk you may have in your freezer. Appropriate solid foods or infant formula may be added, even if it's a bit sooner than you may have preferred.
Adventures in Tandem Nursing: Breastfeeding During Pregnancy and Beyond by Hilary Flower is a good reference for women with questions about breastfeeding during pregnancy (http://www.kellymom.com/nursingtwo/index.html). Also, you may want to discuss your twins’ nutritional needs with their pediatrician.