Congrats on your graduation, first of all!
The way I see it, you’ve got two options. One would be to pointedly ask your family, “I’m sorry, can you remind me whose graduation this is?” Because it’s YOURS, not theirs. Your day of
escape triumph and it’s supposed to be about you and your accomplishment and finishing this stage of your journey and all that. So you have every right to look how you want (barring getting kicked out of the graduation for wearing cutoffs and sandals, but w’ev).
Alternatively, you can try to find a “compromise” dress, something you find comfortable and your family will
STFU about find dressy enough. Dressy doesn’t have to mean tight - maybe you could find a dress that has the loose, comfortable fit you want, in a slightly fancier fabric, or you could dress up your comfortable choices with fancy heels and shiny accessories.
This can actually go well with the stand-your-ground approach, in that if you start strong with “this is *my* graduation and therefore my decision”, you can then “soften” it with “but if you’re that worried about my being dressed-up enough I’ll wear [necklace/heels/etc] with the dress I already chose.”
Remember: you’re basically an adult now, so don’t ask permission - open a negotiation instead!
So really, it’s up to you. Are you comfortable with standing up to your mom and brother about this? Stand your ground and tell them what they can do with their body-policing! If you’d rather not deal with the fallout from that, like I said, think comfortable but nice fabric/details, or accessorize with sparkly things.
Good luck, and congrats again! - jadelyn