Cedrella Black

Cedrella Weasley (née Black) was a pure-blood witch and the second eldest daughter of Arcturus Black II and Lysandra Yaxley. She married Septimus Weasley, who was considered a blood traitor by the Black family, and thus was disowned and removed from the Black family tree. Cedrella and Septimus had three sons, one of whom is Arthur Weasley.

Through Cedrella, the Weasley family is related not only to the Black family, but also to the Crouch and Longbottom families.

Horace Slughorn claimed that the Blacks were almost exclusively sorted into Slytherin house, thus Cedrella might be considered an example of a "good Slytherin" for her non-biased views about blood purity.

Notes and references
Cedrella Weasley