She was a princess and she was being married off. When the scourge came she didn't stay and fight, she ran off. When the Forsaken came back around she didn't mediate or take the side of "her people", she stayed hidden.
She has zero standing with the Forsaken. If anything the Forsaken should be hostile towards her, instead of her coming around when they actually needed her, she's coming around now when she isn't needed, but it sure seems the Forsaken are in an unstable situation; which further makes her look like she is trying to take advantage of a situation.
And I'm not even going to bother explaining Sylvanas, if that needs to be explained to you, you need to go back and brush up on all the game's lore and story lines since War3.