Am I being unfair being really annoyed that my sister has bought a dress like my wedding dress?

I’m getting married next year and my sister will be a guest at my wedding. Although we see each other we are not close and never have been. Well she came to see me try my dress on and has has been looking for a dress for my wedding, she found a dress that my friend had as her wedding dress and wanted to get that. I said no way, it’s similar to my dress and it’s my wedding. So she has now bought the shorter version of this dress, low back like mine and same colour as my dress. Am I being unfair being really annoyed? She knows how I feel. But I am deeply upset and my mum seems to think this is ok too. ???