"Julia goes to her sister's home."
Translation:जूलिया अपनी बहन के घर जाती है।
Apparently it is because there is an (invisible) "ko", which means "to" (Julia goes TO her sister's home). As there is an (invisible) postposition here, the words just prior to it convert to the oblique form, hence instead of "ka ghar" it will be "ke ghar", signaling the invisible ko (which my hindi book refers to as a "ghostposition"). You are supposed to use the oblique form before any postposition, and in this case before any "ghostposition" too.
I'm not sure actually. It doesn't help that the oblique case of it would be identical to just having अपनी because बहन is female. It all depends on whether the possessor of the object in the oblique case is also in the oblique case. I would like some native speaker or someone further along to comment. To make a guess: I think it is in the oblique case.
It tells you what purpose a word is serving in a sentence.
For example, in English, both 'I' and 'me' mean the same thing. But 'I' is used when it is the subject and 'me' when it is the object. They may be considered different case-forms of the same word.
Similarly, when a word is the object of a postposition in Hindi, we use the oblique case form of the word.
I don't think 'invisible' words is a particularly good way to look at it, nouns are in one case or another according to how they're used, and 'ghar' here is in the oblique case.
Sure, other clauses that have some noun and 'ko' might be in the oblique case too, but that doesn't really mean there's an 'invisible ko' here any more than there's an 'invisible of' when I say 'that bag is mine'.
You're thinking in English, not Hindi.
Consider how in English we might say, "We go to her house" but also, "We go home." The "to" is implied when we say "We go home".
I think it gets tricky because simply being the object does not equate with the oblique case. Having a postposition (including an implied one) is a better indicator of oblique case (or rather is what what requires the oblique case in the first place).
apni means the sister is Julia's sister. Uski would mean the sister is someone else's sister, not Julia's. Use uski if you want the sentence "Julia goes to his sister's house." In English the 'her' is ambiguous: is referring to Julia as the 'her' or some other female as the 'her'. In Hindi in this instance this particular ambiguity doesn't exist.
I saw an explanation yesterday that I thought explained this well. They said apne/apni/apna is basically saying ' she goes to HER OWN sisters home'. As in it is specifying that it is Julia's sister. Much the same as the person above me stated in another way. So every time i read a sentence that could be his OWN/her OWN/their OWN I always think apna/apni/apne. I hope that makes some semblance of sense.
The feminine form is ki (की) not ko.
जूलिया अपनी बहन की किताब पढ़ती है (Julia apni behen ki kitab padhti hai)- Julia reads her sister's book
Ko (को) is a different postposition altogether and is different from ka/ki/ke. Ko is roghly 'to' while ka/ki/ke are used to denote possession. Eg: जूलिया अपनी बहन को किताब देती है - Julia gives a book to her sister. Here, 'ko' is used in place of 'to' in the English sentence.