heavy spoilers about the end
704 will become the new king of the north, thanks to that, he'll have his memories from his previous life back (except the ones about Geumsun because Hwan "stole" them), so yes, he will remember his little brother, he'll also know that Eunjae is his reincarnation. But because he's a king now, he can't really go to the mortal realm anymore, so he will say goodbye to
No sorry if i wasn't clear, but you know, Geumsun was trying to have Yoon wi memories back for the last 20 years, and when he had an access to the records of the afterworld about Yoon wi, he saw that every memories about him from Yoon wi life were erased. Hwan stole them, the same way he stole San's ones about Eunjae. But he gave back the memories of San, and tried to manipulate Geumsun so he can free him in exchange of those memories, and of course Geumsun refused.
So now 704 has all his memories from when he was still Yoon wi back, except for Yoon wi memories about Geumsun, because it was stolen by Hwan 150 years ago. But the memories about Geumsun since he wake up 20 years ago, he still have them of course, they're more close than ever now, they have each other and are at peace
No sorry if i wasn't clear, but you know, Geumsun was trying to have Yoon wi memories back for the last 20 years, and when he had an access to the records of the afterworld about Yoon wi, he saw that every memories about him from Yoon wi life were erased. Hwan stole them, the same way he stole San's ones about Eunjae. But he gave back the memories of San, and tried to manipulate Geumsun so he can free him in exchange of those memories, and of course Geumsun refused.
So now 704 has all his memories from when he was still Yoon wi back, except for Yoon wi memories about Geumsun, because it was stolen by Hwan 150 years ago. But the memories about Geumsun since he wake up 20 years ago, he still have them of course, they're more close than ever now, they have each other and are at peace

Can someone spoile me please