"Di mana ipar kamu melahirkan anaknya?"
Translation:Where did your sister-in-law give birth to her child?
You should also be able to say: Where did your sister-in-law have her child? It's pretty common to use "have" instead of "give birth"
I think 'Where did your sister-in-law give birth' should also be accepted, other than a child what would she give birth to?
The Indonesian sentence does use the word anaknya though. I suppose stating that she gave birth to a child makes the child the "topic" of the sentence as a linguist would call it, i.e. it makes it clear that the whole conversation from which this sentence was taken was about the child or the sister-in-law, for example. "So then she spent a summer teaching in Korea and working on herself." "Oh, how interesting! And where did she give birth to her child?" rather than "And the doctor said she had to give birth right away!" "Oh, and where did she give birth?" In the second example, the conversation is about the birth itself, so we don't need to mention the child to shift the attention away from it anymore.