Exactly. Not just etiquette, but decent manners and plain decency. It's such obnoxious and arrogant behaviour to wear a white/almost white dress to a wedding. OR a cream one!!! It stinks of trying to upstage the bride, and 'Main Character Syndrome.'
The OP clearly dislikes her sister in law to be (brother's wife to be) and wants to upset her and ruin her day. The sister-in-law has asked the OP to please not wear this dress, yet here she is intending on 'wearing it anyway.' I'd ban her from the wedding if I was the sister-in-law!
As has been mentioned a number of times, the OP is jealous because her wedding didn't go ahead, and jealous that SHE is not getting married, and she's trying to derail her brother and sister-in-law's wedding, and make it about HER.
She asked 'AIBU,' and 83% of almost 2000 people are saying 'yes you ARE being unreasonable,' yet she is continuing to double down and dig her heels in and say 'yeah but yeah but, I am right, so there!' She has even laughably said 'it's my MUM'S wedding really' because she has contributed something to it financially. 😂
AND @princesspicker if people had seen the dress at the beginning, you would NOT have 50% of people agreeing with you, (as you claimed a few posts back,) the amount saying YABU would be more than 83%. It would probably be 93%! The vast majority of posts on this thread are disagreeing with you!
Also, as I have said multiple times, it's always sisters, and sister-in-laws who pull this shit (and mother in laws to the bride!) Really poor, guest-zilla behaviour.
@princesspicker I really hope your brother and sister-in-law are planning a future life together that is 100 miles away from you and your mum. I would be if I were them. I would be giving you both a very wide berth going forward.