What is Sirius Black's mother's name?

Orion Black was the father of Sirius Black.

Orion Black (1929-1979) was a pure-blood wizard and the eldest son of Arcturus Black III and Melania Macmillan. He married his second cousin, Walburga Black, and the couple had two sons: Sirius and Regulus Black.

Orion and Walburga's disillusionment with the Death Eater organisation, Regulus' defection from the group, Orion's strengthening of his house's defences, and his and his son's death all coincide.