It's not about following formalities or the rules of the house.
Let's say you can't stand the personality of your wife's brother. He whines. He talks ill about other people behind their backs. Is a little rich spoilt brat and just complains about everything and is essentially a snob. You need to deal with that person for a few weeks because they are in town for something and they are staying with you guys as your wife is his sister.
How do you, as a male, deal with annoying in-laws staying over, especially when the annoying person is another adult male in your domain?
Simple, you don't get annoyed or get into gossiping about him.
Realize it's a temporary thing and whether making an issue of it is really worth it.
If it's not, you chill and let it be, if it's not then you make your position clear.
But guys don't seem to get in each others face or get bent out of shape on minor things.
That's just the way it is.