Lysine is excellent and should be given daily, even after the sneezing subsides. It's safe to give to your other cats, too. I haven't heard of giving Zyrtec to a cat. I have given Chlorphenamine Maleate when Bonnie has sneezing fits but she nearly always ends up needing to go to the vet. The problem with this stuff is that it is sedating and has a drying effect. I'd recommend taking Luna to the vet first. Even if no infection is suspected, sometimes another kind of antibioitic will help hasten recovery. If the vet agrees that you can give Chlorphenamine, you can buy it OTC very cheaply.
If it's allergies, look around your home and see if there are any triggers like room sprays/air fresheners, laundry detergents, and cleaning supplies that might be causing Luna to sneeze.