"Had his aunt heard me?"
Translation:Hadde tanten hans hørt meg?
I hade the same question. I also have an idea how to answer it, although i may be completly wrong:
It might be, that 'sin' can only refere to the subject of the same sentences. If i am right, than 'Tanten sin' would refer to the subject, which is the aunt herself, which would imply, that she is her own aunt. Which doesnt work of course