Not sure what to do

My little brother is getting married in October and they asked me to be in the wedding as a bridesmaid. My husband and I have had some pretty big financial setbacks recently , it’s been really hard actually . So I haven’t been able to order the dress for the wedding but they will have to make my size so it will take a little longer then usual. It took my sister in laws a month and a half to get there as they had to make hers as well. So I’m running out of time to get it . Then I’m going to need it to get hemmed . I don’t know if I should tell them I can’t be in the wedding because I can’t get the dress at this time . We have hardly have left over money for groceries and I’ve struggled with affording my blood thinners as well as other bills . I don’t really want to back out of the wedding but I have to be able to feed my kids and pay rent and things have been scary lately . What would you do?? Just buy the dress or back out?