Mrs. Hutchinson likely wants to include Don and Eva, her son-in-law and daughter, because this will decrease the chance that Tessie herself will pull the marked paper from the box. The more people who draw, the smaller a chance that each one has, statistically, of choosing the paper which will result in his or her death. I don't think this makes Tessie a monster; in fact, I think part of Jackson's message has to do with the natural, human, panic we feel when we find that our own life is in danger. Tessie isn't a horrible person—she is simply flailing about and looking for a way to save her own life. It apparently has never occurred to her to protest the lottery before because it has never been her own life in danger before. It is difficult to know what any of us might do in a similar situation.
In one sense, the lottery is "fair" in that everyone has an equal chance of drawing the marked paper. However, in a much larger sense, the lottery itself—as a tradition that is outdated and brutal—is completely unfair because it results in the unnecessary death of an innocent person. The narrator tells us that "no one liked to upset even as much tradition as was represented by the black box" that is used for the slips of paper. People tend to hang on to traditions, even when they have become meaningless or have outlived their original use. This tradition, stoning an individual, is now known to serve no real purpose in the community even if superstition once taught citizens that it did. In this way, then, it is terribly unfair. Why should someone be murdered just because of tradition?