I know this isn't the advice you want but I'd just wear the dress. I'm bi and definitely more the butch one in my relationship (my gf lives exclusively in dresses while I'm more jeans and jumpers) and although I feel like a bit of an idiot wearing one I'd absolutely do it for a wedding. It's not about you and your gf and your identities, it's about your sister and her wedding, and women wear dresses at weddings. Your sister probably doesn't want everyone talking about your gf being the only one in pants, her standing out in pictures, attention being drawn away from her as the bride etc. And yeah maybe her fiances family would disapprove and cause a scene and that's another worry for her. But my point is that it's her wedding and she's probably stressed enough as it is, and it wouldn't kill your gf to wear a dress for a couple hours to make things easier on her. I'd feel kinda awkward and out of place in one but if it made someone I care about happy/more at ease during her wedding I'd be all for it. I don't think your sisters wedding is the correct place to "take a stand" with your gf and I also don't think this automatically means your sister is embarrassed of you. It's just typical wedding etiquette with a stressed bride and her conservative in-laws, and at the end of the day it's just a couple hours in a dress on a day that's super important to the bride but doesn't really affect you for any lasting amount of time. I'm going to wear a suit in my own wedding one day, but I'll wear dresses with shorts underneath to others in the meantime.