Congrats!!!! First off be as sympathetic and understanding as possible. Hormones are a terrible thing to control, especially for first time pregnancies. We are trying to adjust more then anything, so when others notice our craziness it makes us feel worse. First and third trimester is the worst.. I think you will remember that from your first go round. Second she will feel the best. Compliments are wonderful.
Best part about my pregnancy is my husband made me feel beautiful for carrying his child, no matter how fat I got! lol Let her tell you how she is feeling, and always answer with "I may not understand all that is going on, because it's not my body, but I will do my best to help you out". Be sensitive and loving and you will do just fine!