YOU SAID:
mirror is my sisters room
INTO JAPANESE
鏡は私の妹の部屋です
BACK INTO ENGLISH
The mirror is my sister's room
INTO JAPANESE
鏡は妹の部屋です
BACK INTO ENGLISH
The mirror is in my sister's room
INTO JAPANESE
鏡は妹の部屋にあります
BACK INTO ENGLISH
The mirror is in my sister's room
You should move to Japan!