I thought your story was beautifully written, and the sense of loss was palpable throughout. However, I did feel that you gave yourself nowhere to go with the story once you'd established the tragic accident, the mother's inability to let go, and by fast forwarding to when she was elderly. I would have liked to have seen more of the rivalry between the boy and his sister as he grows up. Would he try and do something to exorcise her memory? Maybe he becomes as obsessed with his dead sister as his mum? If the rivalry is the heart of the story, think it needs to build towards some form of terrible dramatic climax, otherwise the tension just tails away at the end. Hope this helps.