And Miriam, the prophetess, Aaron’s sister, took a drum in her hand, and all the women went behind her with drums and with dances. And Miriam sang to them:
Sing to Yahweh for he triumphed!
Horse and its rider he cast in the sea.
This passage (Ex 15:20) identifies Miraim as Aaron’s sister only, and says nothing of Moses. It comes from the same source that defines Aaron and Moses’ relationship as brother Levites, not siblings (see #95). Thus in this textual tradition Miriam is not Moses’ sister.
On the other hand, the only Pentateuchal source that identifies Aaron as Moses’ sibling is the Priestly source (Ex 7:20). Exodus 7:14-25 is another Priestly genealogy whose single purpose was to give Aaron—the forefather of the Priestly writer’s guild—a pedigree. This was done by grafting Aaron into the Mosaic line. But it does more than that. This genealogy’s sole focus is, not Moses, but Aaron, Aaron’s son Eleazar, and most importantly his grandson Phinehas, with whom Yahweh makes the last of the Priestly covenants—the eternal covenant of the priesthood (Num 25:13). Making Aaron Moses’ older brother was a rhetorical device employed by the Aaronid priestly writer to heighten the stature of his ancestor, Aaron, over Moses. See also contradictions #91, #93, #95, and #105.
At any rate, making Aaron Moses’ older sibling also makes Miriam Moses’ sister.