It was announced on Friday that Ibrahim Toure, who was also a professional footballer, had passed away in Manchester at the age of 28.
Both midfielder Yaya and defender Kolo are expected to stay with the Ivory Coast team at the World Cup in Brazil.
Yaya's club side Manchester City have been among those to pay tribute to Ibrahim, and Blatter has now expressed his sympathy to the Toure brothers and the Ivorian Football Federation (FIF).
In an open letter to FIF president Augustin Sidy Diallo, Blatter wrote: "It is with great sadness that I learned of the sad news of the sudden death of Ibrahim Toure, the younger brother of Yaya and Kolo Toure, your players engaged in the finals of the World Cup, here in Brazil.
"Allow me, on behalf of FIFA and in my own name, to extend to you as well as the Ivorian Football Federation, my most sincere condolences.
"I am obliged to convey to the Toure brothers, their families and loved ones, as well as the Ivorian delegation present in Brazil, all my sympathy and support these moments of great sorrow and painful separation."
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