While it was cruel of Kai, he was a kid back then and very very young, with his mother and was scared of losing Alicia, which is understandable.
Remember she tried to drown herself when she first came in the world and that wasn't the last time she tried to leave either. Of course, Kai would cling to her, no matter how much of a little shit he may be and she is the only real family Kai still feel likes he still has. Even as much as Alicia herself might not admit it as much properly, Kai is still her brother.