OotP notes: Arthur Weasley is Sirius's second cousin once removed, so the Arthur's children would be Sirius's second cousins twice removed, Hermione's child his second cousin three times removed, etc., but Molly Weasley is independently Sirius's "cousin by marriage" so she must be the previous daughter of someone who married one of his aunts or uncles (or cousins, if Sirius was using the term "cousin" loosely).

Genealogy note: For a great explanation of degrees of cousinhood, see Answer 12 (bottom of the page) here.