What is the significance of these expressions: "Is it the Dunbars?" , "Is it the Watsons?"
To the extent that these have significance, it is just that they indicate that people are somewhat nervous about the outcome of the lottery. They are very eager to know who has "won" the lotttery.
This is the context in which these words are spoken. The people of the town are wondering who has been picked in the lottery. They are asking whether it is one of these families that has been picked.
So the significance here is that these lines show the tension that is present, even if it was not clear to me when I first read it that it was negative. I thought they were eager to be chosen, not afraid...