Hi,
I had very severe MS - was admitted to hospital twice in the first twelve weeks for it. I still have it now (at 26wks) but not as bad - only vomit a couple of times a week, on average.
Anyway, there wasn't much I could do to help myself. Setting an alarm and getting up through the night to force myself to eat something (even a couple of digestive biscuits), meant my stomach never completely emptied (that's when I always felt worse). I wore Sea Bands 24/7. They certainly took the edge off it for me, though by no means cured me. Another thing that really helped was having DH make all the food, so that I needn't smell it till the last minute. (Don't know how I'd have gotten through it without him!)
As for drugs... I was given some - can't remember the name now. But after leaving hospital and doing some research online, I found a lot of reports of potential side effects both for me and for the foetus. Maybe an over-reaction, but I didn't want to take any chances, so stopped taking them. It did mean that I felt absolutely awful, but the one thing that got me through was what my doctor and midwife both told me.
They told me that before twelve weeks, my baby was really very tiny, with an even tinier stomach. He needed very very little in terms of food, and with the placenta not yet fully up and running, I didn't need to worry too much about the quality of the food I was eating. (I.e. if I wasn't managing my five a day, it wasn't that big a deal, so early on.) They also (charmingly!) both pointed out that my baby was a "parasite". Meaning that anything I could hold down, he would take. Yes, it meant I felt crap, but he was fine.
I don't know about your sister-in-law, but it certainly brought me a lot of comfort to know that Baby was looking after himself.