There is a wide range of holistic sinus products available. BHI makes one called Sinus. We also use the Seven Forest herb, Blue Earth Dragon, which your veterinarian can order for you. Olive Leaf extract, which is a natural anti-bacterial, is good, as is propolis. You can also buy homeopathic nasal sprays which you can spray more or less right up/on the nose or use an eye dropper and place one drop on the nostril opening.
I once treated a cat with a really bad sinus problem. After using a variety of different remedies, I tried a product from Natra-bio called Head Cold, which took care of it. Sometimes you have to try a number of symptom-oriented remedies before you find the one that works for your animal.
You could also try immune-building remedies such as herbal formulas that contain echinacea and goldenseal and/or astragalus.
There’s a chance this problem has been caused or aggravated by upper respiratory vaccinations.