"Julia's uncle loves her aunt."
Translation:जूलिया के मौसा उसकी मौसी से प्यार करते हैं।
It can be 'Julias's uncle loves his aunt' only when the 'his' is not referring to Julia's uncle. Eg: 'Julia does not like Peter's family. But Julia's uncle loves his aunt', where the 'his' refers to someone else (Peter), not Julia's uncle.
If the 'his' is to refer to Julia's uncle, who is the subject of the sentence, you need to use the reflexive pronoun 'apni'.