"His sister teaches him."
Translation:उसकी बहन उसे पढ़ाती है।
I am not sure but, अपनी will represent second person but the sentence is first person right?
"Your sister teaches him" is the translation google translate gave for your answer
Your sentence will translate to "our sister teaches him". In the original sentence we're talking about third person and that third person's sister. Hence the translation "उसकी बहन उसे पढ़ाती है।" will be correct.