Get a letter from your school, explaining your situation. It should state that they want to come here to visit you, meet the new child and return back after a few months. Also state that since it has been a long time you all have been away from each other, there is a need of family unification. They have to demonstrate through bank statements, property documents, their jobs and leave approval letters, flight booking itinerary etc. The theme is to demonstrate strongest ties with Nepal than US. It should work out.