Illness is the most probable thing right now; she can't eat the bento due to her having a strict diet.
So, it's going to all tie up with his goal of becoming a doctor. It'll be his motivation and goal to cure her of her illness. The sister is going to be the one whose going to make him question if he can even achieve the grades of going to medical school.
I hope that this ends in a good ending, not a tear jerking, heart wrenching, dramatic ending.