Back when I was planning my wedding, I gave in to the "musts" from my in-laws (my Dad wrote a check and showed up, staying completely out of things). First, given that I was no pure young bride, I suggested wearing a cream or ivory suit that could go anywhere. Oh no! I allowed myself to be talked into the Lady Diana Dress, which was lovely on the hanger, not so much on me. Same with a big veil.
Then my MIL wanted to know how we were doing the bridal table at the reception. She was scandalized when I said I just wanted to sit among our guests, not up on display like an exhibit. That way, our attendants could go sit with their significant others. Oh no! MIL, who admits she can be pushy, had her way.
My BIL wanted to know what we were doing about tuxes. When I was still thinking cream suit, I told him we weren't doing that; just wear a nice suit. They'll know who you are. This time it was him who said the Oh no!. Even DH got into the act; he was scandalized when I tried to say no to him wearing a white tux to stand out (this was a 11AM, fairly casual wedding). That was a "must", he said. I told him it would be obvious to everyone in the church which guy was the groom. I lost that battle as well, and was kind of meanly glad when my fear was fulfilled and he looked like an ice cream man.
I think I gave in because my opinions weren't that firm, I didn't have any family or "allies" to advocate for me, plus DH was the oldest son and that seemed to have a lot of significance wedding-wise. His youngest brother of the tux argument got married a few years later, and it was much smaller a production (but his bride's family ran the show). I just went with a "choose your battle" mindset.
Today, with years of Ehell behind me, I might have held my ground a lot more.