Inés was also godmother to Queen Maxima's youngest daughter Princess Ariane, 11.
Queen Maxima has been left "shocked and devastated" following the death of her 33-year-old sister Ines Zorreguieta, who was found dead at her apartment in Argentina on Wednesday night.
The youngest sister of Queen Maxima of Holland has been found dead in her apartment in Argentina's capital.
'I wish them all the strength they need to cope with this disgusting loss.
"It is presumed that it was a suicide", a spokesman of the Dutch government told the Argentinian newspaper, La Nacion.
Police said she died of an apparent suicide, state news agency Telam reported. I trust that they will be afforded the space and privacy they need'.
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Queen Maxima, 47, has two other brothers, Martin and Juan, and three half-siblings, the BBC reported.
The Dutch royal family did not immediately issue a statement, though Queen Maxima's scheduled state visits to Estonia, Latvia and Lithuania next week were canceled. Argentinian authorities are still investigating the 33-year-old's death but they too have pointed to suicide. She had previously suffered depression and was admitted to a psychiatric clinic in Buenos Aires in 2012, believed to have been suffering from anorexia and paranoia, the NOS added.
Between 2009 and 2011 she worked at the United Nations headquarters of Panama but returned to Argentina in 2012 after a breakup.
Inés and Maxima's father, Jorge Zorreguieta, died previous year at the age of 89.
Jorge Zorreguieta worked as agriculture minister during Argentina's brutal military dictatorship.