(Exodus 15:20) introduces Miriam with a curious title: "the prophetess, the sister of Aaron." The Mekhilta immediately spots the problem. Miriam was the sister of both Aaron and Moses. Why does the verse identify her only as Aaron's sister? Moses was the greatest prophet in Israelite history, the leader of the nation, the man who spoke with God face to face. Being called his sister would seem like the greater honor.
The Mekhilta's answer is both touching and revealing. Miriam is called the sister of Aaron because Aaron risked his life for her. The reference points to the incident recorded in (Numbers 12:2), when Miriam was struck with leprosy for speaking against Moses. Aaron immediately intervened, pleading with Moses to pray for her healing. He put himself on the line for his sister without hesitation.
Because Aaron showed this kind of devotion, Miriam was called by his name. The Torah honored their bond by permanently associating them in the text. It was not that Moses loved Miriam less. Moses did pray for her, famously crying out "Please, God, heal her!" But the title "sister of Aaron" commemorates a specific act of courage and loyalty.
The Mekhilta's reading establishes a principle: you earn the right to be named alongside someone through sacrifice, not status. Aaron was not the greater prophet. He was the brother who stepped forward when it counted. And the Torah remembered that act by inscribing his name next to Miriam's for all time.