The newly released Black family tree, now revealed in its full glory, is the topic of considerable discussion and is a source of considerable information on the relatedness of the various pure-blood wizard lines. Many familiar names show up on that tree, including Weasley (blasted off, of course). Sirius Black comments in Order of the Phoenix, “Molly and I are cousins by marriage and Arthur’s something like my second cousin once removed . . . .” (OP6). How exactly are Arthur and Sirius related?
Since Arthur’s parents’ names are unknown, we cannot directly link him to anyone on the Black family tree. There is one Weasley (not) appearing on the tree: Septimus Weasley,[*] who married Cedrella Black, causing her to be blasted off the tree. Although it is possible that Arthuris related to Sirius through Arthur’s mother’s side (whose maiden name is unknown), it is reasonable to assume that Arthur is most directly related to Sirius through Septimus Weasley.
Birth dates for Cedrella and Septimus are unknown, but we can make a guess at Cedrella’s birth dates. In the chart, siblings are listed in age order. Cedrella was born between her two siblings, who were born in 1915 and 1919. Thus, she must have been born between these years, but probably not in either 1915 or 1919. So, she was most likely born in 1916, 1917, or 1918. I find 1917 to be the most likely date, since that would put her two years from either sibling, a fairly common age difference, giving the time it takes to recover from childbirth, wean a child, and carry another to term.
Septimus was probably reasonably close to her in age. The only spousal age difference that is demonstrated on the chart is for Sirius’s parents, who were four years apart. Admittedly, this doesn’t tell us much about the possible age difference between Cedrella and Septimus. Since their marriage was probably met with some resistance (considering Cedrella was blasted off the chart), I think it can be safely assumed that their marriage was not arranged, and probably not one of convenience. One can envision several scenarios: Cedrella runs away with Septimus because they are in love or possibly Cedrella runs away with Septimus because she wants out of her parents’ house (as Sirius did) and marriage was the best way of doing so at the time. However, this still does not give us many clues as to their ages. Both scenarios are conceivable for teenagers, 20-somethings, and even middle-aged adults (especially considering that wizards live longer than Muggles). For the sake of convenience, I am going to assume that they were reasonably young and approximately the same age—it makes a romantic story, if nothing else.
In reality, Septimus’s age is less important than Cedrella’s anyway. If Arthur is a direct descendant of Septimus, his mother or grandmother’s age is more of a limiting factor for childbirth than his father. I believe there has been some discussion on the age of menopause in witches—since they live longer than Muggles in general, can they also bear children longer? To sort this out, let’s give a few scenarios in which Arthur is the direct descendant of Septimus and Cedrella.
Suppose that Arthur is the son of Septimus and Cedrella. According to the Lexicon, Arthur’s year of birth is unknown, but we can extrapolate based on his oldest child’s age. Again according to the Lexicon, Bill was born in 1970. We know that Arthur and Molly eloped and the implication is that they did so shortly (if not immediately) after graduating from Hogwarts. Arthur would have been 18 when he graduated. Assuming that Arthur and Molly eloped immediately and more or less immediately had Bill, he could have been born when Arthur was as young as 19. This gives us the latest year Arthur could have been born, 1951. It is possible that Arthur and Molly waited to have their first child, but they don’t strike me as the type to have waited long, especially with a war on and the uncertainty of survival. Human instinct is to start popping out children quickly so as to have a better chance of passing on one’s genes. So let’s give them a maximum of 5 years as a waiting period before having Bill. Thus, Arthur was probably born between 1946 and 1951.
If Cedrella is Arthur’s mother, he would have been born when she was around the ages of 29-34. This is very believable, especially if she had other children before Arthur or was married in her 20s instead of her teens. It is possible for Cedrella to have been Arthur’s grandmother. If Arthur’s father was born when Cedrella was in her late teens (around 1934-5) then Arthurwould have been born when his father was in his mid- to late teens. Assuming that Arthur is a direct descendant of Septimus and Cedrella, which is it?
For this choice, we will turn to the family tree. How is Sirius related to Septimus? If you trace Sirius’s descent through his father’s line, he and Cedrella Black are first cousins, twice removed. Sirius’s father and Cedrella Black’s children are second cousins because Sirius’s grandfather and Cedrella are first cousins, having a set of grandparents in common (in this case, Phineas Nigellus and Ursula Flint). Thus, if Arthur is Cedrella’s son, he and Sirius are indeed second cousins, once removed, as Sirius suggests in Order of the Phoenix. It is possible that Sirius had the relationship mixed up, but since this fits nicely with canon and with our age calculations, it is all around the best answer so far.
Is it possible that Arthur was not the son of Cedrella and Septimus? Let’s examine this possibility. If Arthur were instead Septimus’s brother (largely spaced in age, but technically possible), he would be related to Sirius only by marriage. Since Sirius makes a distinction with Molly being a cousin by marriage, we can assume that Arthur is more likely to be related by blood. Actually, any other relationship to Septimus besides son would make Arthur related to Sirius by marriage only. Of course, it is possible that another Weasley man married a Black woman, but one assumes that she would also have been blasted off the tree, and there are no other Weasleys mentioned. It is also possible that this is not the entire Black family tree, but it is all the canon we have to work with. While we can postulate another Black-Weasley wedding not documented in any way on the tree, Occam’s razor suggests that Arthur is indeed related to the Weasley that we do know is on the tree and other evidence suggests that he is that Weasley’s son.
On the subject of Molly, it is interesting to note that she is Sirius’s cousin by marriage by virtue of her marriage to Arthur. However, the way Sirius mentions their relationship, it sounds as though she is related to him by marriage through some other relative as well. Since there is a Prewett listed on the tree and this is Molly’s maiden name, we can speculate that Molly is somehow related to this Prewett—a niece or cousin of some sort? That, however, is a topic for a different essay.
Does it mean anything, that Arthur appears to be the son of the Weasley listed on the Black family tree? It could indicate that other people whose family names appear on the tree are as closely related as Arthur, but it does not have to. Arthur is the only person with family on the tree whose relationship is clearly stated in canon, so any of the other relationships within the Prewett, Longbottom, MacMillan, and most especially Potter families are still open to considerably more interpretation. Perhaps it is something that JKR will elucidate in time.