This is almost certainly not Molly Weasley's mother, as this would make Molly and Sirius cousins - not "cousins by marriage" as Sirius describes (OP6). More likely is that Ignatius Prewett, who married Lucretia, is Molly's uncle.
Lucretia Black Prewett is born
Certainty Stated in canon
Location in Canon BFT: The Black Family Tree
Type of Event Wizarding world