Hi again, Steve,
MS is not hereditary either, so I don't think that's the key distinction between MS and Devic's.
There's definitely a genetic aspect to MS, but that still doesn't mean it's inherited.
Certain combinations of genes (and it's known to be many, not just one) may make a person more susceptible, but they are still not guaranteed to get MS, even if they have the "high risk" genes.
So it's not a case of simple heredity. About 1 in 5 MSers have a known other case in the family. Which means 4 in 5 don't!
The very fact some races are disproportionately affected means Devic's, too, must have some genetic component, without being hereditary. If it had nothing at all to do with genetics, all races should be affected equally.
Although Devic's disease and MS can present similarly, I would have thought misdiagnosis was very rare these days, due to ever-improving imaging techniques. An experienced neuro should easily be able to tell the difference, since Devic's does not produce the classic brain lesions of MS. I'm guessing that 20 years have seen big changes in the quality of images, so when your cousin was diagnosed, 20 years ago, it would have been more difficult.