Feeling really dumb...

I've just read, for the first time, The Man Who Was Thursday by Chesterton. I'm on my second read through. The first time I blasted through to get to the point. I felt really dumb by the end. I didn't understand any of the symbolism. And the only surprise I got right in the plot was knowing the connection between Sunday and the man in the dark room. Now I'm going line by line back through. Very slowly. Can anyone tell me what Gregory's sister is supposed to be?