So Canada or America is in a relationship with another nation. It's not super serious, so when Nation A starts noticing that their lover has some UST for their brother, they're not so much offended as turned on. They decide that they want both the American brothers in bed ASAP. How this happens is up to you.

I am not big on USUK or FrUk but FrUs or UkCan would be totally fine. All other pairings are fair game.

Bonus: Canada tops
Bonus: They decide that they want to do this again some time