If I were you I would be upset too. Are you and your sister close? I think this would be a great opportunity for you and your sister to bond. As long as you two are doing your own seperate wedding themes, ideas, colors, etc... why not plan together? When one starts copying the other, then there might be problems. If you don't like the whole plan together idea, make sure you are letting each other know about major parts so there aren't any doubles. Like the same dress, colors, father/daughter dance song, etc.... best of luck.