Cedrella Weasley is the mother of Arthur Weasley. Cedrella’s marriage to Septimus Weasley caused her to be “blasted” from the Black Family Tree (BFT).
Spouse: Septimus Weasley (see BFT and my notes). Septimus might be the grandfather that gave Ron the Wizard Chess set (PS12).
Parents: Arcturus Black (1884-1959) and Lysandra Yaxley (BFT).
Siblings: Callidora (Longbottom) and Charis (Crouch) Black (BFT).
Children: Arthur Weasley (Feb. 6th 1950-) Arthur had 2 brothers (JKR); Ron's namesake uncle Bilius (PA6) may be one of them (he could also be an uncle on Molly's side). According to Jo, Ginny is the first female to be born into the Weasley family in generations, so we can deduce that Arthur had no sisters (JKR).
"Cedrella" = a rare feminine variant of Cedric, which in turn is derived from the Old Celtic name Caratacos, which means "love".
While it seems very likely that Rowling intended Cedrella and Septimus Weasley to be the parents of Arthur Weasely, this fact is never actually stated anywhere in canon. For a detailed discussion of this topic, check out this essay:
While there is no specific canon mention that Cedrella was a Slytherin, the comment by Slughorn that "the whole Black family had been in my house" except Sirius (HBP4) strongly implies that any Black who attended Hogwarts during Slughorn's tenure, as Cedrella was, would have been in Slytherin.
Cedrella Black's birth year would be between that of her older Calladora (1915) and younger sister Charis (1919), so we list it as midway between the two: circa 1917.