My sister is being a major MOH-zilla, what do I do?

I would just go ahead with the things you want for the wedding. If she continues to act this way I would calmly tell her "I'm going to try and explain this to you one last time. When you got married you made yout choices on how you wanted your wedding. This is my turn and hopefully my one and only wedding and I want to look back at this day and remember it was exactly what I dreamed it would be. If you do get married again you are free to choose any colour and style dress you like, but for now, this is the dress and the colour scheme I am going with." that way you are telling her this is your wedding without being rude about it. Wanting to choose things for your wedding is not being a bridezilla. Wanting to choose everything for someone elses wedding is.