Leia also has powers with the Force.

Star Wars is basically a more accessible version of Dune. In Dune people had precognitive abilities that would allow them to predict the future. Problem is if someone else can foresee the future one person is reacting to what they predict the other will do, but then the other changes what they'll do based on what they predict the first person would do. So it results in kind of a stalemate. Two people with precognitive abilities can't predict each other's actions. Or the actions of those under the other's influence.

So the Sith's abilities are weakened by the presence of a Jedi and vice-versa. Leia's ability with the force (whether she knew she had them or not) prevents Darth Vader from sensing much about her other. Also the Sith's abilities wouldn't be effective in predicting the actions of any rebel following Leia's leadership.