Twin pregnancy and severe morning sickness

(Hawaii)

I'm 14 weeks pregnant with twins, and I have had terrible nausea, fatigue, and dizziness. I had "morning sickness" with my two older children as well, (4 and 2) but it did not start as early as this pregnancy (4 weeks), was not nearly as acute, and had resolved by this time. I've lost 15 pounds since becoming pregnant. I can hold down a sandwich and maybe a cup of yogurt or a small fruit and a couple of glasses of water on a good day. From what I understand, I should be feeling much better by now - what can I do to help myself?

I'm particularly concerned because of the nutritional requirements of a twin pregnancy and the apparent relationship between diet and preterm labor ...any advice would be great. Mahalo!