Aaron was the older brother of Moses, and so Moses’ primacy in the relationship between the two continues the regular theme of Genesis: The older shall serve the younger.
At the same time, in several specific ways Aaron sets the pattern for the life of Moses. Aaron leads Israel in idolatry at Sinai; Moses dishonors Yahweh before the people at Meribah-kadesh (Numbers 20). In fact, Aaron is denied entry to the land because of the same incident (Numbers 20:24; cf. Deuteronomy 32:50-51). Aaron’s de-investiture and death on Mount Hor (Numbers 20:23-29) was a preview of Moses’ death in the land of Moab (Deuteronomy 34). Both died at the ascent of a mountain (Deuteronomy 32:50), but Aaron went up the mountain of death first.
The rebellion of Moses and Aaron at Meribah-kadesh is reminiscent of the rebellion of all Israel at Kadesh-barnea (Numbers 14). Because of their rebellion at Kadesh, the generation of the exodus was denied entry to the land; Moses and Aaron in effect joined that rebellion by their actions at Meribah, and were denied entry to the land. And that connection of Kadesh and Kadesh brings to the forefront another pair of “brothers” who did not rebel and who did enter the land, Joshua and Caleb.