Okay, so thinking ahead. Your brother is told he is hired, contingent upon the drug/urine test. He is told to go to report to xxxx lab testing facility and take a test. You go, you pee, you show your brudder's ID. If you and your brother don't REALLY look alike, they might pick it up here. I bet it has been tried before. Many of those people who work in those testing facilities have very sharp eyes. They might ask for more confirmation. They might actually indicate there was a problem or an issue with the test.
If so, if anything questionable comes up about your brother's pee test, do you really think he's the one who's going to get hired? Nah. They won't fool with it. Throw this baby out, hire someone else. Otherwise, maybe he will be hired, based on his qualifications AND your clean test.
He shows up for work, works a while, and laughingly tells a very sympathetic co-worker, "Hey, I smoke too, (socially) but my brother took the test for me." It goes straight back to HR. He is called in, questioned, lies, is fired. Or tells the truth and is fired. Machs nicht.
He shows up for work and at some point a very sensitive person he works with smells cigarettes on him. Boy, can you ever smell it, when you are an ex smoker and you get next to someone who's smoked recently! They tell the authority, and HR demands he do a retest. Are you going to take the chance of doing this twice? Really?
I think it is very iffy, skeevy and generally a bad idea to do this, though I can't quote an exact law you'd be violating, I'm not an attorney. I'm just someone who has worked with employment issues and people who worked with employment issues for many years. And one overwhelming thing I have learned is that these sort of situations usually do not end well. If anyone else in the world finds out, they will tell on you. If your brother and your sister in law fall out, she will tell on him. People rat other people out. It usually doesn't work well to take a situation under false pretenses.
Frankly, if I were your brother, I'd go looking for another job at another place. This obviously isn't the place for him. As for you, it's not worth the worry and the bad feeling it gives you and the real questions it might possibly raise. How can you feel ethical when you talk to your children? How can you justify screwing the rules? It is not worth it.