Why Moses Feared Og the Giant and Not His Brother Sihon

Now, Sihon and Og were brothers, for the Master said: Sihon and Og were the sons of [Achiah bar Shamhazai]. What was different, that Moses was not afraid of Sihon but was afraid of Og? Rabbi Yochanan said in the name of Rabbi Shimon ben Yochai: from the answer of that righteous one you may know what was in his heart. He said, "Perhaps the merit of Abraham our father will stand for him," as it is written, "And there came the fugitive and told Abram the Hebrew." And Rabbi Yochanan said, this is Og, who escaped from the generation of the flood. And some say, the fugitive is Michael, as is written above in remez 68.