The following content contains spoilers regarding Volume 12 of the Oreimo light novel series.
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Kyousuke takes Kirino on a date to Akihabara using buying a new Eroge as an excuse to take her there. Even going so far to book a hotel room they could stay at. Though, they only use it to play the eroge they bought. While viewing the Christmas Tree in Akihabara, Kirino tells Kyousuke she plans on studying abroad after graduation. Kyousuke tells her that he's in love with someone. Kirino starts tearing up and runs away. Kyousuke chases after her.
Kaori, Saori, and Kuroneko appear in a minivan (decorated with Siscalypse). They've been following the 2 of them for a while, and decide to help Kyousuke chase after Kirino. Saori tells him that she and Kaori have reconciled.
Flashback to the end of volume 10. Ayase confesses to Kyousuke, but he rejected her. Telling her that he's in love with someone else. Ayase cries and kisses Kyousuke's cheek goodbye.
On December 20th, days before his date with Kirino, he calls Kuroneko. He also rejects her and gives the same reason he told Ayase. But, he tells her how he really feels about her. After crying for some time, Kuroneko rips the Destiny Record apart, and becomes Yamineko, promising Kyousuke that she'll cast the biggest curse of her life on him.
Kyousuke catches up to Kirino, and tells her that he loves her. Kuroneko, using the car audio, plays the confession Kyousuke told her about on the phone...which she recorded. Turns out this is the biggest curse she planned on casting on him.
Kyousuke asks Kirino to not go anywhere, and then asks her to marry him. She says Yes. Back in the Hotel Room, Kyousuke gives her the Ring she asked him to buy for her back in Volume 3. They finish playing the eroge they bought and then go to sleep. (Nothing Happens)
The next day, Kyousuke and Kirino bump into Saori and Kuroneko. Both have accepted Kyousuke's decision. They later run into Sakurai as well, and she also accepts and respects Kyousuke's choice. -Kanako had invited Kyousuke to a Meruru Concert she's performing in. During the event, she confesses to him, but he rejects her. She promises to pay him back by becoming a super star.
One day, Kirino tells Kyousuke that they need to perform a Ceremony for Couples. She shows him the Photo Album she didn't show him before. It's full of Kyousuke's pictures that she stole from their father's photo album.
On Graduation Day, Manami calls Kirino and Kyousuke, telling them she wishes to fight with Kirino. The 3 of them meet at a park, and Kirino tells her that she's dating Kyousuke now and starts to taunt her. Manami then punches Kirino in the stomach and starts brawling, but Kyousuke breaks it up. Manami yells at Kirino the same thing she told her years ago "BECOME NORMAL SIBLINGS" and repeatedly attemps to convince Kyousuke to end his relationship with Kirino, however, he refuses. Manami then confesses to Kyousuke as well, but he rejects her and chooses Kirino instead. Manami, realizing Kyousuke's decision is final, gives up and kicks him in the stomach.
Afterwards, Kirino and Kyousuke have their wedding in the same church that Kirino wore the wedding dress at in Volume 9. Kyousuke and Kirino are the only ones there. Kyousuke took care of the Outfits. Afterwards, they Kiss. Kirino then gives back the Ring to Kyousuke. During Christmas Eve, they promised that after Graduation, they would return to being Normal Siblings.
Kyousuke's final decision is that he won't be with anyone romantically. And that things won't go back to the way they were before.
In the Epilogue, Kyousuke and Kirino go to an Offline meeting in Akihabara, where Kyousuke kissed Kirino again. Then Kirino said that they will have another life counseling when they get home. Then Kyousuke thought to himself, "My Little Sister Can't Be This Cute.". -Sena and Makabe have started dating. -Kyousuke and Kirino never told their parents what happened between them.
Kirino told Ayase that she and Kyousuke are/were dating, however, Ayase's reaction is never shown.
Mikagami became close with Bridget at a Meruru Fan Club meeting. They were caught by the Paparazzi when Mikagami dropped Bridget off at her house. However, Mikagami proudly announced his hobby and that he's really only interested in Unreal 2D Girls".
The End of the volume states that the Bonus Novel that comes with the 1st DVD/BD of the Anime will take place 10 Years After the narrative. So once again...we have to wait to see what'll happen.
Source: zpooonzh13 at Mangafox