The Torah (Vayikra 10:12) refers to Elazar and Isamar as Aharon’s remaining sons. Rashi explains that they too were supposed to die because Aharon’s role in the egel warranted total obliteration of his sons. But Moshe’s tefilla helped to atone halfway and save these two remaining sons while Nadav and Avihu got killed.

This is very difficult indeed. Didn’t they die for their own sin? It doesn’t seem just to kill a son for his father’s sin! If so, Elazar and Isamar are alive because they didn’t sin!