Exodus 15:20 (“And Miriam the prophetess, the sister of Aaron, took a timbrel in her hand; and all the women went out after her with timbrels and with dances.”).
Numbers 26:59 (“And the name of Amram’s wife [was] Jochebed, the daughter of Levi, whom [her mother] bare to Levi in Egypt: and she bare unto Amram Aaron and Moses, and Miriam their sister.”)
1 Chronicles 6:3 (“And the children of Amram; Aaron, and Moses, and Miriam. The sons also of Aaron; Nadab, and Abihu, Eleazar, and Ithamar.”)
I would guess Miriam is the sister of Moses and Aaron.