By the looks of it, Fake Geto appears to be an immortal man who went around fathering sons. So they'd be half-brothers. If they're just brothers and not half-brothers, then that means Itadori is partly a cursed spirit (since the guy said he had three parents and one of them was a cursed spirit), which would explain why he's able to handle Sukuna the way he is right now. I would need more details on that, though. It's all so fascinating.
By the looks of it, Fake Geto appears to be an immortal man who went around fathering sons. So they'd be half-brothers. If they're just brothers and not half-brothers, then that means Itadori is partly a cursed spirit (since the guy said he had three parents and one of them was a cursed spirit), which would explain why he's able to handle Sukuna the way he is right now. I would need more details on that, though. It's all so fascinating.