I am someone who really cares about the feelings of others, and I would never encourage you to do something to willfully hurt your mom’s feelings. However, I agree with pps that what your mother is asking you to do is just not right. It’s one thing for a mother to offer her dress to her daughter and to even hope that her daughter wears it. It is another thing entirely to be “devastated” at the thought that none of her daughters will ever wear her gown and to insist that her youngest daughter wear it now that the two elder daughters have passed up the opportunity to do so.
Just as some brides tend to make things “all about me,” your mother is, unfortunately, trying to make this decision “all about her.” As nice a person as I’m sure your mother is, she really is being very selfish and insensitive about this. And she really does need to see that.
It would be one thing if this were an issue of lack of funds, and your parents were saying to you, “Honey, I’m so sorry, but we don’t have money for a gown for you. The only dress we can provide is this one.” That would allow you to say, “That’s OK. I’m fine with purchasing my own gown.” This is all about sentimentality for your mother. And, there’s nothing wrong with being sentimental — until it becomes about controlling others. Because of your mother’s sentimentality, I am thinking that she would rather you NOT wear the dress (and upset her) than decide to alter it, because, if you alter it, then it won’t BE her dress any longer, and she won’t be able to take it out and look at it or even try it on again herself. I think any attempt to touch this dress would NOT be a good idea.
Instead, I think some pps had the right idea for you. What IF you decided to have some bridal portraits taken in your mom’s gown and present them to her as a gift. I’m sure she would love nothing more than to have a beautiful, framed pic of this somewhere in her home. And, the idea that a pp noted — of offering to display your mom’s gown along with your FMIL’s (if applicable) at your reception is also a nice sentiment. You mom may not even like this idea, but she would certainly have to appreciate the love you would be showing by making such an offer.
Finally, maybe there is something that your mom wore at her wedding (the pearl necklace, the earrings?) that you may actually want to wear with a new gown that YOU are able to select? And, if ALL else fails, perhaps you can convince her that fashions come and go in cycles and that the big, puffy sleeves of the 1980s just are NOT something that women are wearing right now for their bridal gowns. However, perhaps that look may return in the future, maybe when one of her granddaughters is ready to marry.
I wish you the best. You really should be able to choose a dress that YOU love.