This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
Once upon a time, this was one of the big Unanswered Questions in the
X-titles. Of course, once upon a time Bernard the poet was a recurring
character in X-Men as well. Dig those groovy rhymes!
Wolverine and Sabretooth were originally designed, most likely by John
Byrne once he got his hands on them, to be son and father, respectively.
Nothing was ever made of this, besides the usual murky hints behind the
scenes. As time went by the relative popularity of Wolverine versus the
great obscurity of Sabretooth (up until recently, he was still a second-
string villain found working for no-name crime bosses in Spider-Man
titles) made such a revelation rather silly in the eyes of Marvel, so
they just shifted the whole thing over to them both just having some
sort of relationship in the past, but of an unspecified sort.
Recently, Wolverine and Sabretooth have been revealed simply to be
former secret agents who worked on the same team with other mysterious
mutants such as Maverick. A blood test performed by some considerate
S.H.I.E.L.D. medical technicians in Wolverine #42 finally gave us a
definite answer: they aren't related by blood at all. Sabretooth once
believed himself to be Logan's father, but that was merely a vestige of
the Weapon X's memory implant procedures.