Weirdly, something similar happened to one of my male relatives in law.
He was getting married and made the decision to have a suit made to measure by a tailor in his home country. Now this sort of thing has a lot of resonance in his culture; they have this old traditional cultural notion whereby you aren't really a "proper man" until you get married and your wedding suit is the first "proper" suit you have as an adult male. So it has lots of meaning and is highly symbolic.
So he has this suit made, and it is beautiful job. Of course, what happens? He leaves it at his parents the night before the wedding and comes home to catch his younger brother trying it on and taking selfies.
He went absolutely ballistic -- like "woke the entire neighbourhood with his shouting" ballistic. The lad took the suit off double quick and I believe my relative groom-to-be chased him out of the house when he was only wearing his underpants.
Now I'm not condoning chasing people out of a house when they are only in their underwear, but I do think it is worth recognising that clothing sometimes has extraordinary meaning for people, particularly when it is connected to life rituals or changes of some kind.
Sometimes a dress, a piece of jewelry, a shirt are more than just pieces of cloth or metal. They become symbolic and emblematic of an event or a time and place; they become almost sacred.
I mean, how many mns have kept a first pair of baby shoes? Or a cot blanket? Or their baby's little identity tag from the hospital? These things have power. To borrow Marie Kondo's terms, they are the things that "spark" something far deeper and more profound than just "joy".
And I think it is the sense that the "sacredness" of the op's wedding dress has been almost violated that's the reason why she feels something is now "less than". It's not just a dress for everyone to try on for a laugh; it's part of an extraordinary moment for her in her life. It has meaning on and above its pretty clothing potential.
What is doubly hurtful is that this extraordinary meaning was not recognised by either her mother or sister.
So YANBU, OP. I think what they did was unbelievably shitty.