Why is everyone not seeing this is a troll post?
Also she wasn't a sith, she was a dark jedi /sith acolyte, she wasn't even a secret apprentice, she didn't have the darth title and Dooku just used her as an assassin. After Clone Wars she became even more distant from the Jedi/Sith. She also wasn't of the sith race, she literally had 0 claims to calling herself a sith (not that she wanted to any way).
You can't just call yourself a sith just because you have a red lightsaber and some force powers, it's not as simple as that, read the Rule of Two (Darth Bane's novels)