One of the germs hidden within Miriam and Aaron was jealousy. Both Miriam and Aaron were older than Moses, with Miriam probably being the eldest. Being older than Moses, in their heart they regarded themselves higher than Moses and were jealous of the greater respect that Moses received.
Miriam and Aaron said, "Has Jehovah indeed spoken only through Moses? Has He not spoken through us also?" (Num. 12:2). Miriam might have said, "I am a prophetess, one who speaks for God. God has spoken not only through Moses but also through me." Hence, the real issue here concerned the question of who had God's oracle. You may be used by God to speak for Him, but you may not have God's oracle.
At Moses' time the oracle of God was with Moses. According to the Pentateuch, God spoke to Moses many times. Whenever Jehovah wanted something to be done, He first spoke about it to Moses. Actually, neither Aaron nor Miriam was God's spokesman; only one, Moses, had the oracle of God. The jealousy concerning this was the germ that caused the "sickness" in Numbers 12. Throughout the generations, the germs of jealousy and rivalry regarding speaking for the Lord have existed among God's servants.
In speaking against Moses, Miriam and Aaron were speaking against the deputy authority of God. Although Moses was younger than Miriam and Aaron, God had appointed him to be His deputy authority, His representative authority. God had called Aaron and had raised up Miriam, but there is no hint that God charged them to be His representative. But God told Moses clearly that He had chosen Him to be His representative authority on earth (Exo. 3:10-17; 7:1). Because of Moses' timidity, God gave Aaron to Moses as his helper (Exo. 4:10-16; 6:30—7:1). However, this does not mean that He appointed Aaron to be His authority. According to God's governmental administration, Miriam and Aaron should have submitted themselves to Moses. But they rebelled.
Miriam and Aaron took Moses' apparent mistake in marrying a Cushite woman as an excuse to speak against him. However, the fact was that they were competing with Moses in speaking for God. They were in rivalry with Moses. This, not the Cushite woman, was the real factor in their speaking against Moses.