Your use of "brother" is apt. There's a Ukrainian joke that goes something like:
"A Ukrainian man and a Russian man are walking together. They happen upon a $20 bill on the sidewalk. The Russian man says, 'Let us share it as brothers'. The Ukranian man says 'No, let us share it equally'".
The only realistic scenario I can think of when your American "brothers" would go to war across the border is if the Canadian government commences war against its own population. Then I could see the US government intervening, or US fighters independent from the government taking sides in Canada.
The problem is that most people think of these scenarios as something that happens overnight, when in reality consent is manufactured over time. There's a reason you don't microwave a frog.
Because the group of men fit to fight such a war would rather rebel against the government than fight a brother war. From lowest recruit to highest general.