Kenyu is your brother-in-law and he has a son, Isagi, and Isagi is your nephew, but Isagi doesn't know that you are his aunt. He thinks you are his dead mother because you look like your sister. Isagi didn't know that his mother had died because he too young to understand anything, he thought his mother was gone or just fell asleep. When Isagi saw you, he was very happy because his mother had returned, even though you weren't his mother.