Your sibling's spouce's family. ie. your brother's wife's parents. In-laws belonging to your brother or sister, but not to you.
People you have no obligation to see or spend time with, but have to put up with on a regular basis at functions or holidays. Though they really aren't your inlaws, they must be treated as such- just to keep everyone getting along.
The fake in-laws are coming to Thanksgiving, again?
I know- putting up with the brother-in-law is bad enough, without his whole family crashing in.