Is it moral to kill civilians, en masse, who are not actively involved in offensives against you? unequivocally, no.
But much of the support for the nuclear attacks seems to stem from the idea that the Japanese would never have surrendered had we not dropped the bombs. Unfortunately, this is highly debatable and the facts seem to reinforce the side of this supposition being false. From what I have seem from multiple sources (which I dont have handy at the moment but a quick google might yield some results and the professors may have some sources on hand), the Japanese were basically ready to surrender with the demand that they are able to keep their emperor in place. But, the U.S. did not want to allow them to do this, they were “spreading democracy”. Of course, the question arises, wouldnt it then be democratic to have an emperor if thats what the majority wanted but thats beside the point.
So, in my opinion, if your sister can show that the Japanese were, in fact, ready to surrender but their conditions were rejected because the US wanted to unseat their emperor for whatever political reasons it may have been, it would change alot of minds on the topic (probably not the teacher by it would at least get the others students to think).