When Moses objects that he cannot speak, Yahweh assigns his “brother Aaron” to be his spokesman and prophet (Exodus 4:14; 7:1-2).  The next time Aaron is identified as Moses’ brother is in Exodus 28, where he is given the garments of glory and beauty to approach Yahweh, and the phrase “Aaron your brother” appears again in Leviticus 16:2.  In all these cases, Aaron as brother functions as mediator, the one who approaches the master, whether Pharaoh or Yahweh.