Amamiya Akira is the brother of Amamiya Yuuko. Akira is not blood-related to Yuuko.
About YuukoAkira lost his sister in an earthquake (coincidentally, the earthquake that killed Akira's sister was also the earthquake that killed Himura's sister).Thus, in hopes that Yuuko would replace his biological sister, he adopted Yuuko, who is a splitting image of his biological sister.
Akira was initially very happy and treated Yuuko like a princess because he had Yuuko who would replace his biological sister. However, as time went by, he slowly realized that Yuuko was not his sister and began to despise her. Thus, he began to abuse her.