"They are my brother's sons."
Translation:वे मेरे भाई के बेटे हैं।
from the Tips & Notes of this lesson:
Whenever a doer or receiver of a verb takes the oblique case, all the descriptive words attached to it take the oblique case as well (since all those words together are considered the subject/object).
Example: मेरे केले में – In my banana Here, the subject is मेरा केला (my banana), and hence, both मेरा and केला take the oblique case. का/की also influence the preceding entity to take the oblique case.
There are no mistakes here and has nothing to do with formality, in Hindi when a noun has a postposition after it (Those being: का के की पर में को से) it puts the noun into the oblique form. The oblique form in Hindi: masculine singular nouns become masculine plural (though only gramatically, you are still only talking about one brother here,) masculine plural nouns go into a special form where ओं gets added onto the end, feminine singular nouns stay the same, and feminine plural nouns also go into the ओं form. Some examples: My brother = मेरा भाई, On my brother = मेरे भाई पर, My brothers = मेरे भाई, On my brothers = मेरे भाइयों पर, My sister: मेरी बहन, From my sister = मेरी बहन से, My sisters = मेरी बहनें, From my sisters = मेरी बहनों से. Make sense?
I see. My mistakes were then 1) not treating "ka" as a postposition but as some special type of word of its own, 2) for that reason, not realizing it requires oblique case, 3) forgetting that the form ka/ki/ke depends on the possession and not the possessor. And all examples so far with 's/ka have been with possessions like apple and book, which don't change in oblique singular.