I have 10 days worth of cephalexin 500 mg left from my husbands skin infection. Can I take this for a sinus infection? I am allergic to morphine

No. I applaud your efforts to save money / avoid waste and am glad that you asked first. While Cephalexin may work for some of the bacteria that cause sinus infections it is not the drug of choice so by taking it you would run the risk of side effects for naught. Allergy to morphine is not a concern in this context.