Clear Skies! Vol. 1 (manga)
Obinata Taiga, editor of a science fiction magazine, one day discovers there's a new addition to the family! Out of the blue, his older sister's married, and his new brother-in-law is Shu Asuo, one of the authors he edits and a high school classmate. Taiga's dead set against it, but then his sister suddenly disappears! Because of that Taiga ends up taking care of his little brothers, and living with Shu!