Gideon and Fabian Prewett were Molly Weasley's brothers. Both were members of the original Order of the Phoenix. They were killed by Death Eaters, including Antonin Dolohov, but not before fighting "like heroes". Molly gave Harry Fabian's watch as his seventeenth birthday gift.
Septimus Weasley was the Weasley children's paternal grandfather. He has never appeared in the books and is known only from the Black Family Tree, which shows that Cedrella Black was disowned for marrying him. Like his son, he is considered a blood traitor.
Uncle Bilius is never seen during the books, but according to Ron, whose middle name comes from him, he died after seeing the Grim (an omen of death). He is described as being the "life and soul" of parties by Fred and George. Ron also mentioned that he never married "for some reason".
Ignatius, a member of the Prewett family who was married to Lucretia Black of the Black family.
Molly Weasley's second cousin, whom the family rarely talks about, is an accountant and a Squib. He has a daughter named Mafalda, who was originally planned by Rowling to possess magical powers and be sent to stay with the Weasley family before attending Hogwarts. She was also to be a rival to Hermione - excelling at schoolwork. Rowling axed her, though, as her "function in the plot didn't fit and instead she was replaced by Rita Skeeter."
Lancelot, a cousin of Muriel, was a St Mungo's Healer who found the fact that Ariana Dumbledore never received medical attention at home suspicious.
Apolline and Monsieur Delacour, parents of Fleur and Gabrielle Delacour. First seen in book four when the champions were to be seeing their families before the final task in the Triwizard Tournament. In the final book they arrive to the Burrow two days before Fleur's wedding and are described as "helpful, pleasant guests".