I was your daughters 20 years ago.
What people choose to do with, or put on, their bodies is completely up to them and doesn't require comment.
They're adults, they get to choose whether or not they attend. Why are you trying to persuade them otherwise knowing full well how it makes them feel? It gives the impression you're putting the feelings of others above your daughters and that's a sure fire way to lose their trust. Adults or not, it's nice to have someone in your corner.
"They are polite kids and so don’t respond in the way that I would if someone was commenting negatively about my appearance" ... I'm struggling to understand this comment. It sounds like you're saying you'd fiercely advocate for yourself, but you don't for them.
Unless they've asked you not to say anything for fear of making an uncomfortable situation even more so - advocate for them! They've opened up to you about how they feel and a continued lack of support speaks volumes.
It may sound silly to some, but I lost a lot of trust for certain family members as a teenager for this sort of thing. And I'm sure much like OP they were absolutely trying to do the right thing.
But - in my experience - it's a reflection of who they are and how they wish to present themselves to the world as people. The family members are making unnecessary comments for the sake of a cheap, lazy joke without thinking that they're ridiculing who they are as people, it is not just about clothes.