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Why do you think it is a sinus infection?

That looks like canker to me. Return her to where she came from. Canker can be treated, but it infects the whole flock. It is a chronic disease spread through birds using the same waterers. You don't want that in your flock.

Definitely keep her separate! She should be in quarantine for several weeks. She clearly has something going on with her because that swelling is not normal. Does she have any odor about her face? Any plaques in her mouth or throat? Any open wounds anywhere around her head or neck?