If I were you I'd hire a tiara and deliberately OTT bling for the occasion! Personally I don't mind what anyone wears on the 25th but I do like to wear something a bit more special just because....but my idea of special is someone else's casual. Maybe take DD aside and tell her that, for you, it's part of your day to wear something which makes you feel good, and everyone else is free to wear what makes them feel good too. Be assertive but non-confrontational, emphasise that you're sorry others feel uncomfortable with what you wear, but that isn't your intention at all. I don't think anyone, family or not, should be dictating your dress code!