Aamir's mother's sister is his aunt is the same as Aamir's mother is his aunt's sister! All the english translations of these sentences are so clunky and unnatural. No one would phrase it like the 'correct answer' unless they were intentionally trying to confuse someone. I've gotten this translation 'incorrect' 5 times now, simply due to the fact that the translation that they are asking for is so bizarre and unnatural.
I'm not mother tongue Hindi, but it might be because of the complicated kinship structures in Hindi that it comes across as clunky in English. For example बहन can also mean cousin, and there are two types of aunts through the mother's side: the mother's sister (मौसी), and the wife of the mother's brother (मामी). But that's just a shot in the dark.