Melania Macmillan, a member of the pure-blood Macmillan family, married Arcturus Black and had two children, Lucretia and Orion. She was grandmother to Sirius and Regulus Black.
Melania's two children were likely born when she was in her mid-twenties (on the assumption that she was about the same age as her husband who was 25 when Lucretia was born and 29 when Orion was born).
melan = Gr. black