Can I take cipro (ciprofloxacin) to treat a sinus infection?

Yes. Cipro (ciprofloxacin) is a broad spectrum antibiotic. Not everyone would agree that it is a "best drug" for a sinus infection. There may be others that wold be considered better for this. The question is "do you really have a sinus infection?" a sinus infection can be a viral infection in the upper passages with pain and discomfort or allergies with fullness/congestion and pain or frank bacterial infection.
Cipro (ciprofloxacin) Sinus. I'm not a big fan of Cipro (ciprofloxacin) for sinusitis. There are many other antibiotics that are a better choice, unless cultures have shown pseudomonas. In patients who are immunocompromised, such a recent chemotherapy or HIV infection, Cipro (ciprofloxacin) may be a good choice.