Can cephalexin help with a sinus infection ?

I guess, that the answer is a bit of yes and no. Cephalexin is not recommended for the treatment of sinus infections (sources: many). That is not to say that bacteria in your sinuses will not respond but that its use (as a broad spectrum antibiotic) may lead to the build up of more resistant bacteria which is an undesirable outcome. If your sinus infection is viral in nature, then no antibiotic will be beneficial.