The sentence is very non-Arab-esque. It is literally dull. We do not say "ta3baan". We say "ta3eb"
Is that formal arabic? Because I think they use ta3baan to teach the fus7a and not amia
That's the point. "ta3baan" is amia, not fus7a. They say in their introduction guide that they are trying to teach spoken Arabic, not the Standard Arabic.
How do you know this is "my brother is a tired accountant" vs "my accountant brother is tired" (like if you have 2 brothers, one is an accountant and one is a flight attendant, and you are telling your new partner what to expect at the meet-the-family dinner tonight ... vs the given correct answer, which seems more like "I have one brother and, as accountants go, he tends to be the tired type")
This sentence is my brother is a tired accountant because the tired corresponds to the accountant. To say my tired brother is an account, it would be: (My brother, the accountant, is tired) .أَخي المُحاسِب تعبان