Arthur is "something like" Sirius's second cousin once removed. This describes a relationship where one person's great-great grandparent is the same person as the other person's great grand-parent.
Therefore, Septimus Weasley is Arthur's father. Septimus's children would indeed Sirius's second cousins once removed. Assuming the Black family tree Jo auctioned is complete, this relationship fits the facts and can be explained no other way.
(c) hp-lexicon
дерево
И тут я узнала, что такое second cousin once removed:
en.wikipedia.org/wiki/Cousin
Аааа, мой мозг...