In Numbers, chapter twelve, verse ten, the sin of Mariam and Aaron in questioning the authority of Moses was that they did not fear God or respect God’s Word through His prophet Moses. Moses was the mediator of the old covenant, just as Jesus is the mediator of the new, as stated in Hebrews three, two through six. God spoke directly to Moses. Therefore, Moses’ word was the authoritative word of God. Though Mariam and Aaron were leaders in Israel, they had no right to challenge the authority of Moses.
God showed them that they were not on the same level as Moses, so believers today have no right to put themselves on the same level as scripture. Christians are followers of Christ, and we have the privilege to access the Word of the Lord. The Holy Word was orchestrated through the prophets to provide us the opportunity to absorb and obey it. However, some people compare themselves to other leaders believing they are capable of the position of the ones they see before them. These people fail to understand that God ordains who fits the role He has for the faithful, not the leaders themselves.
The situation with Mariam and Aaron with her becoming leprous is because of what they saw Moses do: he married an Ethiopian woman, as stated in verse one of the same chapter. How come Aaron did not become leprous like Mariam? Only God knows. However, based on the circumstances, jealousy played a role on behalf of Mariam. Why would God punish one and not both?
God sees right through the heart of everyone. After all, He made us. We cannot fool Him with how we feel about things and situations with others around us. Especially trying to cover up the real reason we behave in such a manner toward others is just a waste of time. Particularly toward servants of God will only bring discipline and judgment, which could range from mild to severe if we are not careful in what we do because it eventually leads to sin. We all know by now how God feels about that.