Besides medication you can also try giving her a stress ball that she can squeeze if she feels tense. This may help her to stop squeezing her tummy when she feels like things are going wrong.
I recommend going to your doctor and asking for a prescription for a medication called Zofran (Ondansetron). It is an antiemetic that can be taken for all age groups. (We give it to babies when they come in through the ER, so it can be given to an 11 yr old).
Also, another medication called antivert (meclizine) may be useful for motion sickness as well. But I think you need to be 12 yrs old to be prescribed it. I will ask some physicians when I go to work later for exact age restrictions.
The overall family of anti-nausea drugs is known as antiemetics. There are different types of antiemetics, and thus far you have only tried two of the antihistamine type: Dramamine and Benadryl.
Antihistamines are often prescribed for motion sickness because they are effective in preventing nausea in that particular case. However, because your sister is experiencing nausea before the “motion” of the journey even begins, I am not surprised that they have not been effective.
Your sister may well find relief with a different type of antiemetic drug that will be more effective on her nausea. Other major families of antiemetics include 5-HT3 receptor antagonists, Dopamine antagonists, NK1 receptor antagonists, and cannabinoids. You can find a detailed list of examples in each of these categories on the page I linked above.
Depending on the country you are in, you may be able to find one of these over the counter, or you may need a prescription. Either way, though, given her age I advise you strongly to consult a medical doctor to ensure you get something that is properly dosed for a child of her age.
Obligatory Disclaimer: I am not a medical doctor, I just grew up surrounded by them.
I hope that you are able to find a treatment that is less motion-sickness-specific that will help your sister travel home comfortably!
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