I hope your sister isn't trying to drink anything cold during or in the days after oxy infusion? That is usually what brings on the feeling of glass shards in the throat. She should stick to warm fluids only, and perhaps ease into room temperature. The mention of seltzer makes me wonder.
She should also wear warm gloves and socks even if it is relatively warm where she lives.
I never had a strong metallic taste that I noticed, but I also ate a lot of spicy food. Things like ginger candy can help, and ginger settles the stomach. The idea of holding a peppermint candy in your mouth while drinking is brilliant!
I also never had outright nausea, but they gave me Emend and steroids in the pre-meds, and the times my stomach felt kind of queasy I found an onandsetron helped. That and eating a couple of saltines to avoid having an empty stomach.
I think it is very important to keep ahead of nausea. At my place, they made sure we were stocked with three different medications from the start, and said if the first one doesn't work, take the second after x time period, and so forth.
BTW, there's a lot of information here on the effects of radiation on women with rectal cancer. You might not want to share it with her when she's already struggling with this phase, but it is probably good to be forewarned for later, because there are some steps she can take if she knows before going into it. Doctors tend not to tell women about this, we have discovered.