The English test

The head foreign teacher gave us, the head head teachers an English test today to test how good our English is apparently. There were 50 questions and half of them confused me. I asked the other foreign teachers what the answer to the second question is. I ( Chinese ) said it's " brothers- in- law", Andy( English) said the same thing, Jasmine(Canadian) agreed with us. Devin( American) insist that it is " brother- in- laws". I meant to ask K, but ended up with being quiet. I mean, this is a grammar test, but I'm just so desperate.Can anyone tell me what the fuck's going on here? Is it "brothers- in- law" or" brother- in- laws"? It upset me so much today because I felt like my English is just awful and maybe I should quit and stop being an English teacher! Maybe, the school should stop giving us stupid tests like this so that it doesn't drive everyone crazy!