When you are a twin I imagine you would be quite fascinated by the behaviour and reactions of other twins. When you are not a twin (I am not a twin) it is equally fascinating to see how two people who look so much alike behave in such individual ways.
Grace Lin is the author of Where the Mountain meets the moon which is a much longer and far more complex novel for middle Primary students and yet with Ling and Ting, even using a very controlled vocabulary, she still makes a very satisfying set of short stories perfect for a newly confident reader.
In the first story of six the two little girls are having their hair cut. Ling is able to sit still but Ting fidgets and squirms. The scissors slip and Ling and Ting are not exactly the same anymore. The final story is a recap (sort of) of the preceding five stories but Ting mixes up the events and this will make young readers smile.
Here is the author web site with some activities. Here is an excellent detailed review in the New York Times.