The Portugal captain is rumoured to have paid a surrogate mother to give birth to Cristiano Jr back in 2010.
Despite the youngster being unaware of his mum's identity, Ronaldo's family have insisted that he has the choice to find her once he gets older.
"Cristianinho sometimes asks about his mum," Elma told Diario de Noticias. "Once we told him that she was in Heaven, but my brother and my mum didn't like that and asked us not to repeat it.
"We say now that his mum went travelling. He goes silent, forgets about it and doesn't ask anymore."
Ronaldo, who is in a relationship with model Irina Shayk, has sole custody of the child.