Responses
In a way korch is with both the brothers now, sona physically and sien mentally like the comment said. Also as sien is a priest who will never marry someone he will really live all life reminiscing kirch. While sona have kirch imprisoned for eternity and kirch already broken beyond hopes. So in some way they all with each other, a truly dead dove do not eat ending

I was sick of being blue-balled by the wait for the english translations (couldn't find raws either past 13) so I went ahead and bought the rest of the damn raws on official.
Spoilers if you want to know the rest of the story after ch 13:
When the brothers were imprisoned as kids their prisoners took Sien to be trained as a priest at their church bc they considered him a child of god. He liked the special treatment until he found out one of their rituals included self-sacrifice from their followers to achieve holiness. One day the high priest tells Sien he's to be the next sacrifice and that it's a great honor to do so, but he can avoid it if he kills his brother instead. Sien tries to stab Sona while they're sleeping back in the cell but he can't do it and gets caught mid-stab by Sona. Sien begs for forgiveness and Sona forgives him. From then on Sien devotes himself entirely to Sona and swears absolute faith and loyalty to him.
Cut back to present day -- Sien is looking for Kirch but can't find him. He's still imprisoned with Sona while the latter fucks him daily. When Sien confronts Sona on Kirch's whereabouts Sona tells Sien that he forgets his faith and that he's been indulging in too much sin and desire. Sien begs Sona for forgiveness for his lack of faith and Sona tells him his sins can be forgiven if he does one thing -- offer to Sona what Sien's heart desires the most (Kirch).
When Kirch finally escapes Sona's study he tries to look for Sien and sees him getting on a carriage. He hides behind a tree and finds out that Sien is leaving for the parish (priest community). The people accompanying Sien worry that he's leaving too soon, but Sien tells them that there is nothing left for him at the palace. Devastated by his departure, Kirch retreats to the church heartbroken and defeated. Sona confronts him and tells Kirch everything he's done with Sien was done so he can shoulder Sien's sins -- so when Sien finally gives up on Kirch all of Sien's sins would be absolved and he would be able to enter priesthood as a pure man. Kirch goes mad at the revelation and asks Sona to kill him, saying he knows Sona hates him and wants him dead anyway. However Sona hugs him and tells Kirch "You were the last tribute offered to me for Sien's perfection, how could I not love you?"
Half a year goes by and Kirch is nowhere to be found at the palace. His fellow guardsmen continue to look for him, though one of them thinks that Kirch could've followed Sien after he left for the parish. Instead we cut to Kirch getting railed by Sona back at his private study. Kirch's mind is completely broken and he hallucinates, thinking he's with Sien and repeatedly calls his name. Sona tells him that he can continue imagining him as Sien, but ultimately Kirch is his and his alone, and one day Kirch will be reborn into someone who loves only Sona as well. Present day story ends here.
The epilogue is a flashback after the brothers were rescued -- Sien is looking out from his window at flowers falling from the tree in front of him. He sees Kirch down below speaking to an official about accepting the knight commander position in light of saving the princes. The official tells Kirch that Sona will be crowned soon and not to focus too much on the second prince (Sien). Later Kirch sees Sien outside picking up fallen flowers from the tree he was watching earlier. Kirch picks Sien up from the ground and brings him up to the tree to see the flowers blooming from the branch. Sien tells Kirch he was trying to pick flowers for Kirch to get his attention. Kirch tells him he doesn't have to do that because he'll come whenever Sien calls and he'll always be there to protect him. Little Sien hugs him closer and we fade out.
THE END
PS: The author notes that the title 'Patera' is a spoiler for the ending -- its original meaning is that of an offering plate used in Greek religions. Basically Kirch gets placed on the patera as an offering (to Sona) to absolve Sien's sins and become pure before entering priesthood.
Ending was kinda semi-depressing and plot was rushed, but art is good so I'll give it a 6/10.