Aholm, Locke pretty much covered every issue I had with your second sentence. I think the re-write is good, but its lacking the image/emotion that came to my mind when I read this:
Anyway, here's mine:
Terona tried to ignore the nebulous figure on the edge of her vision, lingering to make certain that she carried out her duties. She stitched closed the long gash on her patient’s arm and—due to her dwindling supplies—didn’t have a choice but to use her teeth to cut the colourless thread.
“Why are you sad, girl?” Isreh asked, forgetting his manners.
[For context, the next sentence states that Isreh is Terona's brother, and the sentence after that shows that Isreh's question was directed at the patient]
A thin smile crossed his face as he remembered that today he was known as Rasmus.
Anyway, here's mine:
Terona tried to ignore the nebulous figure on the edge of her vision, lingering to make certain that she carried out her duties. She stitched closed the long gash on her patient’s arm and—due to her dwindling supplies—didn’t have a choice but to use her teeth to cut the colourless thread.
“Why are you sad, girl?” Isreh asked, forgetting his manners.
[For context, the next sentence states that Isreh is Terona's brother, and the sentence after that shows that Isreh's question was directed at the patient]