Hi and welcome to the Chiari forum.
It is possible to have a sinus issue along with Chiari as it is in close proximity to where the herniation is and with all the area being tight from compression it is not uncommon for us to also have sinus issues....that said, it may be the chiari itself as season changes can make how we feel much worse.
After finding ur herniation, did u have other testing to rule out related conditions like syringomyelia (syrinx), tethered cord, ICP, POTS, Ehlers-Danlos, sleep apnea, or if u have an obstruction to CSF?
Many times it is the related conditions causing more issues then the herniation and there are far too many Drs that do not feel this condition will cause us the issues it does.
We do have a list of Chiari Drs for u to use to research.....see the health Pages -
http://www.medhelp.org/health_pages/list?cid=186