"Her father's sister is her aunt."
Translation:उसके पिता की बहन उसकी बुआ हैं।
Since I'm really no expert on grammar it's difficult to unpack the terms used on sites that try to explain the usages of words. I keep mixing up the use of उसके and उसकी here, and I think it's because I'm missing a key element that's described here: https://en.wikipedia.org/wiki/Hindi_pronouns#Possessive_Pronouns "each is inflected to express the grammatical person of the possessor and the grammatical gender of the possessed". Does this mean that because "father" is masculine, उसके is used because he's the "possessed", and similarly उसकी for "aunt"?
Let's say we are talking about Neha's aunt here - 'Julia's father's sister is Julia's aunt'.
While translating 'Julia's father', Julia is the possessor and 'father' is the possessed. Since 'father' is masculine and pluralised for respect, we need to use के and it becomes जूलिया के पिता.
While translating 'Julia's aunt', Julia is the possessor and 'aunt' is the possessed. Since 'aunt' is feminine, we need to use की and it becomes जूलिया की बुआ.
While translating 'Julia's father's sister', 'Julia's father' is the possessor and 'sister is possessed. Since 'sister' is feminine, we need to use की and it becomes जूलिया के पिता की बहन.
The whole sentence is thus जूलिया के पिता की बहन जूलिया की बुआ हैं।
Replacing the noun जूलिया with the pronoun वह, the choice between what form of वह, among उसका, उसके and उसकी, we need to use is similar to the choice between का, के and की above.
So, the sentence is उसके पिता की बहन उसकी बुआ हैं।
Since "उसके पिता की बहन" in its entirety is the subject of the sentence, I believe that would mean that the father's sister is her own aunt; i.e., the aunt's brother is also the same aunt's father. Ick. So, probably not unless you are talking about an Egyptian Pharoah's dynasty. I'm not 100% sure here whether अपना can refer to a subject that's in the oblique case at all.