Ookuninushi is Susano'o 's son-in-law. You need to read more Japanese history. Ookuninushi is a Kunitsukami (land god), Susano'o is an Amatsukami (heavenly god). In the past, Japan was under the Kunitsu's rule with Ookuninushi basically their king. His name literally means "Great master of the land". The Amatsu live in and rule heaven (Takamagahara). One day, Amaterasu sent her grandson to descend and conquer Japan, thus, sparking the great war between Kunitsu and Amatsu. Susano'o is kind of an oddball since he didn't side with his sister and fellow Amatsu.
Historians suggest that the war between the gods is symbolic for the war between the Yamato tribe and other tribes, since the Yamato tribe worships the Amatsu while other tribes worship the Kunitsu. In the end, the Yamato conquered Japan with the Japanese Emperors claim that they are descendants of Amaterasu, thus, having divine status.
