Absolutely! There are a number of essential oils that might help:
peppermint Mitchum, spearmint, chamomile, lavender, and digestive spices such as ginger, coriander, and cumin are possibilities. All have potential for helping relieve her nausea but your sister really needs to find which essential oil she has an affinity for - which she actually likes the smell of in other words.
A drop or two on a tissue to inhale as the nausea strikes, and blending a few drops in Auroma's pure shower gel, shampoo and conditioner, and body lotion for her morning shower.
As long as her pregnancy is healthy, small amounts of essential oils used for a couple of weeks at a time are less risky than many household products.
I would also suggest she drink her choice of peppermint, spearmint, chamomile or real ginger/lemonade - traditional remedies for upset tums - practice meditation and get regular massage from a qualified therapist specialized in pregnancy massage.
Your local healthfood store should have samples available to smell so you know which to buy - then give Kevin a call and he can direct you to the most cost-effective choice.
Essential oils can be extremely useful throughout pregnancy, during birth and beyond. Also ask Kevin which of his books he recommends. Once essential oils become part of your daily life you'll wonder how you managed without them.
Hope this helps. All the best!