The choice seems to be: chaos and tantrums, or a happy child in an unfamiliar setting. I'm not saying that you should let your kid wear a stained t shirt and a diaper, but really, you'd think that the rules might be bent just a little for those who are 2 and under and those who are 85 and over.
What is she comfortable in? It seems that after a long trip, lots of relatives, and new surroundings, it would be reasonable to let your child wear something comfortable. Could you buy her beautiful new pajamas? Would she wear leggings and a pretty sweater?
I took my daughter to a wedding once. You would have thought the pretty shoes I tried getting her to wear were made of porcupine quills and cactus needles. We were far from home and I was afraid we would not make it to the reception because of all the screaming (her) and crying (me). I understood that she was nervous, anxious, tired and who knows what else. I let her wear her sneakers. Not a soul noticed.
It doesn't seem very likely that your child, at age 2, is saying to herself "wow, I'm going to make my mamas really mad by refusing to wear this dress. I hope this ruins their day. Ha ha! I'll show them who's boss." Now, at age 13, that would be a real possibility! But at age 2, your baby doesn't understand formal wear, dinner, in-laws, Thanksgiving, silverware, wine glasses and social etiquette. She only knows that this dress is itchy, or she senses tension in the air, or the dress is unfamiliar and therefore not comfortable, or that her legs are cold, or that this is way more fuss than buttoning up overalls over her favorite Frozen shirt.
It sounds like she's very perceptive, and very aware of her surroundings, and very aware of sensory issues. Perhaps it isn't a disability, but it's her nature. Some kids wear anything that's thrown on them, some kids have to have things just so. My son wasn't particular about anything that he wore except for his shoelaces. If they weren't equal bows, life would come to a screeching halt, and it was a pretty easy fix. But that was just a small thing with him, not an overall problem. With my daughter, it was more of an entire body problem. Clothes often didn't feel right or comfortable.
I encourage you to be honest with your in-laws. Tell them with the long journey, all the new faces, tiredness, etc., that your littlest angel won't be wearing formal wear, and you hope that her introduction to her new family will be as comfortable as possible for everyone. Her beautiful face will be all that anyone remembers!