Tsuna died at age 40 due to a chronic illness. Long before his death, Tsuna once said to Byakuran.
"You know, Byakuran? I want to be born into the world with the same family... and I really want to have a brother. Then, in that world, I am not a Vongola boss" Said Tsuna
Before his death, Byakuran take Tsuna soul before the soul would disappears, Byakuran sent that soul into a parallel world, the world Tsuna desired. "But remember Tsuna... there is a price you have to pay for that life..."
Then, Tsuna unconsciously, he has been born into a new world, in the same family, as male baby, and he have a brother.
Sawada Nana in this world is different from the one in his world. His body was also not the same as in the previous world.... is this the price he have to paid? But fate seemed to hate him. He would never be able to escape from the mafia world.
Even so... He love them all.