@tutepani:
Your dad is the petitioner and the SPONSOR. You are the JOINT SPONSOR since he doesn’t have enough income to sponsor not because he doesn’t live in the US. Your dad needs to fill in i864 as the petitioner and the main sponsor. You will fill in another i864 as a joint sponsor.
While he doesn’t need to have enough financial income (that’s where you come in as a joint sponsor), he does need to be domiciled in the US or have a plan to live in the US before his beneficiaries can be granted an immigrant visa. In other words, either he has to come to US earlier or with your brother for your brother to be able to immigrate. For NVC process, either he has to be residing in the US or show documents about his plan to move to US.