The 40-year-old is expected to announce his retirement at a press conference on Thursday, after captaining Juventus to a seventh straight Serie A title and fourth consecutive Italian Cup triumph this season.
"I really don't think it's time for goodbye," Veronica Buffon said.
Buffon comes from a sporting family, and both his two sisters Veronica and Guendalina played volleyball for Italy.
"The more I see him on the pitch, the less he seems to me like a player walking into the sunset," Veronica, 43, told Turin daily Tuttosport.
At the family's hotel in Tuscany the player's most important trophies and jerseys are proudly displayed, starting with the first he wore at Parma.
And Veronica said space had not yet been made for the one he will wear for his final game expected to be Saturday when he makes his 640th and final Serie A appearance against Verona.
"I was a volleyball player, I played in Serie A and in the national team, and I quit quite young, at 27, because I had nothing left to give, I felt deflated and felt the need to start a family. I had a thousand reasons for wanting to quit, but I wonder what the reason Gigi should do it," she added.
"Because he's 40, so what? We're talking about the goalkeeper who was a protagonist in the seventh consecutive Scudetto won by Juventus, and the fourth Coppa Italia.
"Not to mention that in the Champions League, at the Bernabeu, he put in a fantastic performance. Gigi is a UNESCO world heritage, and trust me I've played high-level sport.
"I'm his sister, I love him very much and I'm biased, but I really don't think it's time for goodbye. Gigi's eyes don't give me the feeling he wants to stop.
"I've told him this so many times, but Gigi is a good guy he just laughs and almost always replies: 'maybe it's better this way'.
"As his sister and a former athlete I want to hear him say: 'I'm stopping because I don't have it anymore', but that's not how it is.
"I don't see him in America or China, Gigi is someone who has always played to reach goals, to win."
Italy's most capped player with 176, the 2006 World Cup winner could make his international farewell on June 4 in a friendly against the Netherlands.