I really enjoyed chapter 13 because of the role MA plays in it. We finally see her character develop, up until now she has been very quiet, but in chapter 13 we start to learn a lot about Ma. The one thing i noticed, or the pattern that seemed to keep reappearing was that Ma was always helping one or more of the family members and didn't ever need any thanks, she seemed used to just being there, and being helpful. "Ma helped Granma in and got in beside her." Although just a simple example, we see small gestures like this go unnoticed by the rest of the family.
And as I had mentioned early we also see Ma develop a lot more as a character, or at least show the reader more about Ma. She seems to be a very interesting character, simply because she never has any problems, or doesn't even let us, the ready, the third person omniscient reader, what's going on in her head.