@AlbertoT: If the brother that stays on Earth would transmit his own heartbeat to the brother on the ship, the brother on the ship would also think that the heartbeat of the other brother is slower than it is as measured on Earth.

That is the case for all inertial systems in special relativity. Moving inertial systems always perceive the other as if time was moving slower there. It seems strange and illogical, but it's true.

Therefore, you need general relativity which explains it by the acceleration that is inevitable if the brother on the ships ever wants to come back.