The 19-year-old woman and her 32-year-old male accomplice were arrested near Barcelona on Tuesday on suspicion of belonging to a terrorist organization, La Voz de Galicia reports.
The two suspects, both Moroccan-born Spanish residents, are thought to have sworn loyalty to Isis and its leader Abu Bakr al-Baghdad. They are also charged with attempting to recruit jihadists from among the people they know.
The male suspect rejected the charges while the 19-year-old woman admitted to the charges but said she had ceased carrying out such activity five months earlier.
The woman also confessed to having told her four-year-old sister "positive things" about Isis in a bid to indoctrinate her, judicial sources said.
Spain's Interior Ministry believe the two suspects set up an international terrorism recruitment network and had "constant contact with active members in Syria".
The pair are also thought to have glorified terrorism and to have distributed multimedia material to teach new recruits how to use arms and explosives.
Like other European nations, Spain has been grappling with a growing number of jihadist cells on its territory and radicalized Muslims leaving to fight for Isis or other Islamist groups in Iraq and Syria.
Con esta última operación antiterrorista, ya son 100 los yihadistas detenidos en 2015 y un total de 175 desde 2012 pic.twitter.com/M7pgpOWiv9— Ministerio Interior (@interiorgob) diciembre 8, 2015
The arrest of the latest two suspects on Tuesday brought the number of suspected jihadists arrested since the beginning of the year to 100, the interior ministry said in a statement.