Fetus measuring 1.5 inches at 8 weeks. 1 yolk sac visible. Could it possibly be twins and only half of one was visible making it appear to be large?

Unlikely . The fetus is much smaller than 1.5 inches at 8 weeks - although the entire fetal sac is much larger than the fetus itself. At 8 weeks there isn't much to see, but it's unlikely to be a twin pregnancy.