In June, Spain's National Court revealed a lawsuit alleging "corruption and swindling" by Neymar, his father and the player's former club Santos.
Now the same Madrid court has revealed it will also investigate Nadine Gonçalves da Silva Santos, the player's mother, who holds a 50 percent stake in the company that runs her son's affairs.
The case has been brought by a Brazilian investment fund, DIS, which owned part of the transfer rights for the player when he was at Santos.
They claim they were cheated in a murky transfer deal that Barcelona and Santos originally said had cost €57 million ($65m, Ł41m) but that Spanish authorities say cost Barca at least €83.3 million.
DIS believe an agreement between Neymar and Barcelona saw €40 million paid to the player's management company to stop other clubs bidding for him and that they were deprived of a cut.
The Spanish court asserts several top European clubs including Manchester City and Real Madrid had indeed been interested in signing the player.
Former Barcelona president Sandro Rossell was forced to resign when the allegations emerged.
Meanwhile Neymar helped Barcelona win La Liga, the Copa del Rey and the UEFA Champions League last season.
N and N is also being pursued by Brazilian tax authorities, and a Brazilian court last month froze $47 million (€42 million) in assets belonging to the player.