Amy, you might want to follow this up, as siberians are more prone to several degenerative eye conditions than a lot of breeds. Learned a new one recently- a siberian owner I met on the beach told me that blue-eyed siberians have much higher incidence of glaucoma - her dog ended up needing one of her eyes removed.

Found this on the i-net:
"Uneven pupils or anisocoria, may be of no pathological significance, but pronounced difference in the size of the pupils can be symptomatic of an organic lesion such as an Aortic aneurysm, tumor, enlarged thyroid, glaucoma and intracranial pressure."

Our local Adopt-a-Husky rescue in Ohio runs eye clinics twice a year - maybe you can locate one out there & schedule Nebo to be seen by a canine opthamologist, just to make sure he's ok & put your mind at ease. Hope this info helps.