I'm so sorry to hear that your niece is having to go through such a rough time! I know there are a few women on this board who had TERRIBLE m/s, and had to be hospitalized due to dehydration, and I'm not one of them.
I have heard however, that anything with ginger in it (tea, ginger snaps, there are even pregnancy lollipops that you can get to help with m/s, and I believe ginger is one of the ingredients) can help for some people, so it's probably worth a try. It didn't help me with my nausea, but everyone's different. I also agree with Tami to try to make sure she has something to drink (not necessarily water), ALL the time. If she sips on something all day long, she's much less likely to become dehydrated, than if she waits to feel thirsty, and gulps down a glass of water (which will probably end up coming back up).
And I also agree to just tell her to really be aware of how she's feeling--if she feels dehydrated (dry mouth, not peeing much), she should go to the hospital to be checked out for sure. And try to make sure she takes those prenatals! When she's not likely to get vitamins and minerals from other sources if she's so sick, she needs them more than a lot of pregnant women at this time.