My sister says she has optic neuritis. Are family members more likely to get the same thing?

Not likely. Optic neuritis, a disturbance of the conduction of the visual signals from the eye to the brain, is usually associated with multiple sclerosis or also as an isolated problem unrelated to anything else. There is no inherited reason for these conditions.
Optic neuritis. No, optic neuritis is an inflammatory problem affecting the optic nerve. It is not an inherited problem, although underlying neurological diseases or systemic inflammatory diseases that could be an underlying cause of optic neuritis can run in families.