Ronaldo, 41, was named in Portugal's starting line-up for the round-of-32 clash with Croatia in Toronto.
Speaking to Portuguese broadcaster SportTV before kick-off, Katia Aveiro said she believed it could be her brother's final appearance for the national team.
"From the information I have, they can say goodbye.
"Enjoy it while it lasts. It's not today that they're saying goodbye, but it's soon. I believe this is their farewell. Enjoy it a lot. It will be difficult to find someone like him.
"I'm talking about the national team. The information I have, from a reliable source, I believe this is his last dance. For intelligent people, those who like football have to like Ronaldo. Those who don't like Ronaldo don't like football. They're the ones who lose out."
Aveiro pushed back on any suggestion that criticism of her brother had affected the family.
"He's been tearing it up for over 20 years. Do they think we're not happy? Look where we are, the Aveiro family, and where we came from.
"We've achieved so much, considering where we came from, after the struggle we went through, what my brother went through, the achievements my mother had. Do they think the criticism will interfere with our happiness? Never.
Aveiro also noted it could be Luka Modric's final game for Croatia, having played alongside Ronaldo at Real Madrid for six trophy-laden years.
"We have to enjoy these moments. It's the last dance for two players, both for Croatia and for Portugal."
She reflected on the pride the family feels at reaching this stage of the tournament.
"The most important thing is to enjoy these 20-odd years that we've lived, won, and come so far. I'm incredibly proud. I was in Qatar, I'm here. It's a tremendous pride."
Aveiro said she remained confident of victory.
"I'm confident and we'll smile at the end. Ronaldo is great and confident, less nervous than us. I felt a good energy and confidence. For us fans, it brings comfort. We can trust."