Esemori and the mother are clearly related. The mother is HIGHLY likely to be the girl who survived the suicide attempt. No big surprises there. Then the boy who did die will likely be someone with a close connection to Esemori, possibly a brother or twin or something.
Then Reiji's father will somehow appear back in the picture to create drama.
Then Reiji's brother will commit suicide. He'll be the only real suicide in the series. That one is kind of transparent insofar as the brother is a clear throwaway character.
That's all just predictions on how this will go.