YTA.

Sorry, maybe it's because I'm not 'american', but this just seems unnecessarily mean of you, and cruel.

Yes, it is a special dress, and you have a chance to give your sister an amazing memory and let her have just the sliver of a feeling of what it would be like to have a dress made by her mother for her.

Your being extremely unkind. Jealously guarding this dress and not letting your sister share I part of your mother. It will sit in your closet for how long? Gathering dust and only living in memories?

You had a chance to do something beautiful for your sister, and I'm sure your mother would have wishes for you to be kind and share the dress.

I dunno. I'm from the Balkans and we see things differently in that part of the world. I would gladly lend my wedding dress to any of my cousins of they needed or wanted to wear it.