Usubeni Yamada chose the maiden name "Yamada" as she was told it was a common family name for Raen and she has no recall of her actual family name or contact with any living family members. Her son Mikhail Yamada inherited this name by default as she was unmarried when he was born. The suggestion was given to her by a traveling merchant when she was leaving Coerthas during The Calamity.
It is completely unknown if she shares any relation to anyone with this surname, as it was chosen as an alias to sound like a "believable" Raen name so that authorities would not assume she was Eorzean-born.