Moses’ older siblings, Miriam and Aaron, began to speak against Moses because of his Cushite wife. The reason or motive for their slander is not clear, but their charges appear to be unfounded considering the fact that in the end God punishes Miriam.
Their reason, or the smokescreen, for their verbal attack against Moses could have been that Moses married a foreign woman. Ethnic purity was important in Israel, for example, they are commanded to drive out the Canaanites, yet within the Israel camp foreigners were present and welcomed. Foreigners could join Israel in the covenant blessing by following the statutes and being circumcised. Foreigners could even celebrate the Passover along with Israel. (Exodus 12:48-49; Lev. 24:22; Numbers 15:14-16: 15:29)
The account seems to indicate that the attack on Moses by Miriam and Aaron is based more on a motive to undermine Moses authority. They said:
“Has the Lord indeed spoken only through Moses? (or “…spoken only to Moses?”)
– Numbers 12:2
It would seem that Miriam felt her position and importance was threatened since, Aaron and his family already have a significant place in the Israelite camp.
The Lord immediately called all three of them to stand in front of the Tabernacle where he defends Moses' ministry and rebukes Miriam and Aaron. The Lord then left Miriam with leprosy until Moses interceded for her.