I can sympathize with Edwina to a point but after awhile everything the Viscount did she took as love and not him just simply courting her. He said from the very beginning when all of her suitors were trying to impress her that he couldn't give her love but companionship he even explained that he would never really be around if they got married and had kids because of his duty. Kate gave up a lot for Edwina. The only reason Kate put effort into Edwina and Anthony was because Edwina said she loves him and she wants him, so Kate put her feelings aside as if they mean nothing and helped her sister at every possible turn. At the beginning she tried to discourage the union. Edwina had blinders on and chose to ignore all the obvious signs and just blamed her sister as if she was plotting against her the whole time. I don't think Kate has a very high opinion of herself since her father and mother had no station in society and she is not Mary's biological daughter. I think she always feels like the help most times and the only way prosperity can happen for the family is through Edwina.