Nothing to fear about brother. You are going to get graduated and it is essential for you to share your happiness with your family members. Your family members are your sponsors who have paid money to the University for you to get an education. The American Embassy will surely grant visa for graduation provided you really studied and utilized what an american university has to offer.
The Visa officers will ask certain question about financial condition and your parents must be transparent. They should prepare some answers to convince the officer that they are financially well off and show intent to return back to Nepal. If you follow this then they will hopefully get a visa.