How does Kate not know?

I really was confused why they made it seem like Kate didn't know who his wife was but talked to her without a problem in the doctor's office telling her that she is sleeping with a married man with out being scared. She was at a few partys that the Fosters were at so she should know it was her. It almsot looked like she didn't even play the part right cause at the dinner table she still didn't act scared only the husband did. Just seemed so odd.