Maybe Yuuchan fell into a coma after some bad accident, the manga does mention he began soccer practice around that time. The mom might've had something to do with the accident, which could explain why she is the way she is... idk bro
My favorite story so far. I really loved how it sits right on the edge of supernatural and hauntingly real. Whether the young boy was spirited away by something otherworldly that eventually claimed the whole family, or whether the mother simply lost her mind after the tragic death of her son; the ambiguity is perfect. It gives you just enough to chew on, leaving the story lingering long after it ends.
I'm thinking that Yuuchan is... dead, but the mother tries in vain to cover the memories (hence pretending Yuuchan didn't exist), until the family can't take it anymore, or getting tempted by the robot maybe possessed by something?