Once a friend of mine asked the same question and it was really difficult for me to define criteria that would define a true Nepali. As I delve into it, I was looking for a common criterion that would comprise all the Nepalis.  I couldn't find a common denominator to include everyone (including you) : people from mountain to hill, east to west, all ethnic groups and all dialects.  As I was pondering a thought popped into my mind:

 A NEPALI IS THAT PERSON WHO HAS NEPALINESS AND NEPALINESS IS THAT FEELING THAT EMANATES WHEN ONE IS NEPALI .

It is paradoxical and doesn't give any discrete answer; I, in my moronic regard, feel it as a rational opinion. I believe anyone can be a Nepali if one meets my afore-mentioned criteria regardless of the Nepali citizenship certificate issue.