I'm disappointed and surprised that SIL's medical team isn't making suggestions. Not sure how much they have been told about SIL's continued and increasing nausea, as well as weight loss. It is quite concerning, but I am treading carefully. Hopefully my BIL will ask about these issues now that I've suggested them, including the possible in-home nurse to provide an IV for nutrition & hydration.
I can easily symathize with SIL not wanting to consume fluids that might increase her nausea, which annoys BIL. To me (who has tried to support my D when she had extreme nausea, as well as myself), I can certainly ID with not wanting to eat or do ANYTHING that might make me feel worse, including drinking ANYTHING.
Hope if BIL communicates about the severity of nausea, dehydration & nutrition concerns, more assistance will be forthcoming.