Nails fuchhi, Don't make the situation too complex. All those big things about caste and creed will become trivial when parents will realize that they should let their kids do what will make them feel happy. Of course, you should be confident enough on yourself to feel what you are doing is right.Then only you can go about convincing your parents. Nepalese society is conservative but its not that much that it will ostracize a brahmin who marries a chhetri these days. Your arguments would be valid if we were like 40-50 yrs back, we have changed although not to the extent of our likeability. Nepali society does accept intercast marriages these days.