In general cases, getting Visa is just matter of luck of person and mood of consular. Inviting a brother for graudation and Bhai tika has very some chances of success, if you can prove good financial background and strong ties back in nepal. Last year I witnessed myself , a cousin and his wife, of a friend of mine got visa for convocation. We all were surprised. so in my opinion it worths giving a try. But as a general rumour, if denied once, then it may become a sort of problem in future if he/she has to come to the USA in future.