Katia Aveiro, 33, spoke out as she denied stories about the woman's identity - but did not explain when or how she died.
"There is no woman calling him. There is no mother, no telephone calls, nothing. His mother died. The baby doesn't have a mother," she said.
When the former Man United ace said last July that he became a dad, he said the mother wanted to stay secret and that the boy, called Cristiano Ronaldo Jr., would be under his "exclusive guardianship."
Portuguese media claimed the £200,000-a-week ($320,000-a-week) Real Madrid star, 25 - who is dating Russian underwear model Irina Shayk, also 25 - paid a surrogate in the US to have the baby.
But in December, a British newspaper claimed the mother was a 20-year-old British student who had a one-night stand with the striker. She accepted £10 million from the sport's top-paid player to give up her rights as a parent, the paper claimed, but then changed her mind and called Ronaldo, demanding to see the boy.
The star reportedly told friends he was worried she would try to take his son away, but Aveiro dismissed the UK reports as "nonsense" and said Ronaldo's mother Dolores, 55, was raising the child as her own.
She refused to discuss surrogacy claims but told a Portuguese magazine, "The boy is ours. I'm not going to say how he arrived with us, but I guarantee he's my brother's son, my nephew, from our blood. His mother is my mother, who is the person who is with him 24 hours a day."
The family said the baby was born in the US on June 17.
And while critics accused the soccer star of jettting around the world with his girlfriend, leaving the baby behind, the striker's sister said, "He's the most important thing in my brother's life."