So, my brother is going to get married. His girlfriend(it isnt totally official yet so I wont call her his fiancee)has 3 brothers. Would they be considered my brother in laws?
No. They're just your sister-in-law's brothers... It's like saying would you call your sister-in-law's mother, your mother-in-law... No...
Well, technically no. But you could consider them that if you liked.
I guess so