She and her mom are mother and son and you and she are husband and wife.
You have to provide a certificate of mother-child relationship and a certificate of marriage to do so.
I personally think that let her mother in Japan to find an intermediary to do it can be, it is best to be better intermediary, or the Ministry of Justice of Japan recognized by the Executive Secretary, the success rate is higher.
GOOD LUCK!!!!
GOOD LUCK!