This was a great scene with Mary running into Colin's room and giving him a piece of her mind. 'A nice sympathetic child' Mary was not, but what she really did here was to treat Colin like he was an ordinary boy and not a sick invalid and demanded that he take some responsbility for his own behavior.
I think it may be in the next chapter, but Martha's mother is quoted saying something like, 'The two worst things for any child are always having their own way or never having their own way'. Easy to understand this concept, but of course the balancing is the difficult part.
For both Colin and Mary learning to care about others and like themselves goes hand in hand, learning to love and becoming more lovable and in turn loving themselves.