I'm not going to invite my in-laws for christmas
For what reason would you not want to invite them?
I want to show my in laws that they are not my priority.
Even if they aren't your priority, not inviting them for Christmas is just being rude. They may be upset if they were expecting to be invited.
No one cares about them, so why should I do it?
You should care because they're still your spouse's parents. Whether you like them or not they are family. Its pointless to just burn a bridge if you can peacefully maintain the relationship.