Regardless of current standing, Asajj was once and should now be, Sith!

None of this expanded universe and cartoon stuff is canon anyway.
Give it a rest. Ventress is not a Sith. You can only be a Sith by being one of the officially designated "two" (Master and Apprentice).

For example, Dooku only became Sith when Maul died. He was a dark force user before, but no sith. He became a Sith once being officially named Darth Tyranus.

And of course after the death of Vader and Sidious the siths were eradicated.