Jack is "touched" by his sister's presence at lunchtime in the field. Why is this? What does he notice about her?

Jack is touched when he notices how seriously his younger sister takes her responsibilities. He watches her carefully set out lunch with the hands of a little girl but the mannerisms of an older one. Jack watches her and is struck by her intensity of purpose giving him his lunch. He can see that she, too, is troubled by thoughts but does not let them interfere with her responsibilities.