My (F21) sister in law (F24) wants my mom's (F54) wedding dress and it's putting a strain on our relationship.

My brother (M26) announced yesterday that he and his fiance (F24) set a date for their wedding in a little less than a year. My brother has always expected things from my parents, and I don't get along with my sister in law. They told my parents to buy all their alcohol, and that they drive all the guests in my deceased grandfathers' sportscar.
To make matters worse, my sister in law texted my mom and asked if she could have my mom's wedding dress. It really rubbed me the wrong way, because I'm the only daughter and the dress should be a family heirloom and given to me when it's my time to get married.
I told my mom years ago I would love to have her dress one day and alter it into my own dress. Mom only said my sister in law could come try it because she was confident it wouldn't fit her (she is overweight).
The only reason my sister in law wants it is to save money. It doesn't mean anything to her, she doesn't even like my parents (she called my father stupid in front of me). I'm ****** and annoyed.
This situation is putting a strain in our already shay relationship. How do I go about this?
TDLR: My sister in law (F24) is demanding, spoiled and rude and expects to have my mother's wedding dress for her own wedding in less than a year to save money. My brother (M26) and her have already expected to drive all the guests (they live like 6 hours away) in my deceased grandpa's car, and buy all the alcohol for the wedding. And now she wants what's rightfully my (F21) heirloom. How do I go about this when our relationship is already shaky?


Source.