Why would my father s girlfriend where a fancy white long dress to my wedding?

This is what she bought online. https://www.promgirl.com/shop/dresses/viewitem-PD2281701 And I wore a simple dress. https://www.promgirl.com/shop/dresses/viewitem-PD2280169 People that didn t make it to the ceremony thought she was the bride. My father didn t say a thing, but my mother was furious. My father s... show more This is what she bought online. https://www.promgirl.com/shop/dresses/viewitem-PD2281701

And I wore a simple dress. https://www.promgirl.com/shop/dresses/viewitem-PD2280169

People that didn t make it to the ceremony thought she was the bride. My father didn t say a thing, but my mother was furious.

My father s girlfriend was a guest, she has been married twice before and should know the etiquette at such an event.

I guess she felt the need to upstage me at my own wedding.... BUT WHAT RIGHT DOES SHE HAVE TO DO SO?
Update: @Anonymous, We are planning a honeymoon party at the place where we will be on our honeymoon to make up for what she did.... She won't be invited to that, or any future weddings in our family. If she exercised some wedding etiquette and didn't upstage me, she would have been invited to more events, like... show more @Anonymous, We are planning a honeymoon party at the place where we will be on our honeymoon to make up for what she did.... She won't be invited to that, or any future weddings in our family. If she exercised some wedding etiquette and didn't upstage me, she would have been invited to more events, like baby showers... etc
Update 2: @Foofa, there's only 5 years age difference between us. I guess for her it was all about putting me in my place at my wedding, I refuse to be put in my place at my wedding. Thank you Foofa, Slowly everyone at my wedding now knows what kind of woman she is, even my grandmother on my father's side detests her... show more @Foofa, there's only 5 years age difference between us. I guess for her it was all about putting me in my place at my wedding, I refuse to be put in my place at my wedding. Thank you Foofa, Slowly everyone at my wedding now knows what kind of woman she is, even my grandmother on my father's side detests her now. I guess I'm Okay.
Update 3: @p , My mother and grandmother pulled her aside and had a few words with her, from what I know she's not allowed to show up at my grandmother's house in the future with my father... Her loss
Update 4: The wedding pictures looked horrible, I didn't look like the bride, so the company that did the pictures altered the colour of her dress to dark brown matte. We made sure she and my father got some of the photographs. If they don't like our changes, that's too bad.
Update 5: @clayton, I'm not, My mother, my grandmother (father's mother) and myself are taking control back. The company that did the wedding photos made her dress a horrible shade of brown. I'm sure my father will be happy.... NOT
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