First things first: It’s not actually even clear that the “sister” mentioned early on in Exodus — in the basket narrative — is actually Miriam. The text only refers to her as Moses’ “sister.” So where do we get the name “Miriam”? Not until after the crossing of the Red Sea in Exodus 15:20. “Then Miriam the prophetess, the sister of Aaron, took a tambourine in her hand, and all the women went out after her with tambourines and dancing,” it reads. 

At various points in the Torah, God is portrayed as speaking to Miriam directly — in the same way he does to both Moses and their brother Aaron (via EWTN). And according to the Talmud — ancient Jewish writings that supplement and interpret the Old Testament — she was the only reason Israel had water in the desert. Ultimately, her “merit” led a miraculous well to follow them (via Chabad.org), which is why the people of Israel don’t complain about being thirsty until shortly after her death (Numbers 20).

Unfortunately for Miriam, her undoing proved to be what the modern crowd likes to call “racism.” In Numbers 12, she (along with Aaron, who gets off comparatively easily) complains about the fact that Moses is married to a Cushite instead of a Hebrew woman. God isn’t impressed and strikes her with leprosy, and she’s cast out of the camp until the skin disease goes away. And while she’s eventually let back in, she doesn’t get any more dialog in the text until her death (via Jewish Women’s Archive).