In Shemos 24:1, God tells Moshe to go up to Him with Aharon, Nadav, Avihu and the 70 elders. They do, and they catch a glimpse of God from afar. It seems strange that of the 4 sons of Aharon, only Nadav and Avihu got to attend this event. Why weren't Elazar and Itamar invited?
Based on Tanchuma, Rashi claims that by gazing at the throne of Hashem, they sinned, but due to the solemn atmosphere of the covenant, their death sentence was suspended until they brought the alien fire (Vayikra 10:1–2). Regarding why their two other brothers didn't go, the Artscroll Chumash brings the following explanation:
Tur comments that God purposely did not send Elazar and Issamar, Aaron's younger sons, with the group so that they would not incur the death penalty, like their brothers. Had they, too, died, Aaron would have been left childless.