I'm going to try to give the short story of this! Both of my parents passed away several years ago and my brother was married this summer. His in-laws are very nice. He spends a lot of time with them in absence of our own parents. Which is fine, except he now insists that I spend my Christmas with them before we go to our family. While they are very nice, they are not my in-laws. Am I nuts to think this isn't right?

It's not necessarily about whether it's right or not. It's about what you want to do and whether it might be worth it to suck it up and visit for an afternoon because it makes your brother happy. Remember, family is valuable wherever you may find them. However, if your brother is demanding that you visit them at the expense of you spending time with other family members you would rather be with, you may want to gently remind him that although you like his in-laws and might spend the occasional holiday with them, it is not what you would like to do most of the time.

Tell your brother to get a life. You'll spend your holidays however you see fit.

May wish to check dates, this was Dec 18 before christmas, and the OP never has gotten back as to what happened, and most likely not a issue till next christmas