I don’t think they clan will be massacred seems like the grandma might die though. It would be too heavy for a shounen if they massacred them i think especially for the tone they already set.
I get a darker vibe from this story then what seems to be going on. At first I felt the rental brother was using the kid for money, but I'm not so sure anymore. After that, when they were going through his backstory I felt as if he had left something equally as important rather than just not...