Perhaps the strangest thing about the situation with your twins is that it is the girl, not the boy, who lags behind in language development. Twins tend to be slower than singletons in language development—especially if they were significantly premature as yours were—and boys more so than girls.
Your own behavior will be critical in helping both of them learn to talk. Even though your life must be stressful and complicated now, as two children that age are a handful (I know because I have boy/girl twins), it is important that you find time to talk to them, read to them, play little games with them, sit with them and share their toys with them. At their age, “announcements” of what you are going to do will help them learn new words. You could say, for example, “Mommy is going to get your milk out of the refrigerator now,” stressing words you want them to learn.
I think you have it right in saying that your daughter bites her hands out of frustration. Not being able to talk and tell you what she wants may be part of her frustration. The best way to handle that kind of behavior is to stay alert to a rise in her tension level and get her out of a trying situation before it reaches the biting level. That’s not easy to do, but usually young children give advance signals before they hit another child, bite themselves, have a crying fit, etc. So if you can spot the rise in frustration and quickly intervene (pick her up, offer her a different toy, etc.) you can hopefully prevent her from reaching the point at which she bites herself. With that approach, you are not “giving in to her” but are simply helping her gain control of herself.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.