I can indeed sense frustration for Aka Ka and Oshiro clan not merging despite careful plans. However there is still living heiress of Oshiro clan who can by right of blood inherit it; Emiko. So to make sense to most scenario and make up Aka Ka victorious, they IC executed Oshiro clan daimyo in field of battle. But before they could 'claim' all land of Oshiro, Takeda informed them that he has a heiress coming over claim it and then he suggested clan merge in the meeting earlier (Gunba x Aka Ka x Oshiro). Of course Aka Ka can refuse the offer and then proceed take over Oshiro clan lands (but I suspect the clan lands turn neutral and need taken over by Aka Ka per square and contest them with Hachisu clan who is likely more than eager to grap Oshiro lands now without daimyo on it).