Sister Carmen Longatela Latas, 82, and Sister Carmen Vazquez Lamela, 88, have been indicted by a court on suspicion of tricking or forging mothers to give up their children.
The nuns deny the accusations.
According to The Times a police operation named Baby is investigating irregularities between 2005 and 2011.
The convent has a contract with the local government in the area to take care of single mothers who struggle to look after their children.
A Brazilian woman, Cilene Domingues Lourenco, stayed at the convent after giving birth.
She claimed that she was forced to sign a consent form giving up her child.
Lawyers involved have claimed that criminal gangs have been involved in the selling of children.
