This part of the story comes after the Hutchinson family has been selected from all the other families of the village. It is important to remember that at this stage we still don't know as readers what the true horrific significance of winning the lottery actually involves, so we are left to wonder why it is that Tessie is trying to argue so much about various aspects of the lottery. She first tries to say that Mr. Summers did not give her husband enough time to select a paper and then is told firmly to "shut up" by her husband.
When Mr. Summers asks Bill if there are any other households in the Hutchinson family, Tessie, well aware of what is going to happen next, quickly interjects with the following piece of information, even though she knows that now her daughter is married into a different family she can't be included:
"There's Don and Eva," Mrs. Hutchinson yelled. "Make them take their chance!"
Tessie reacts in this way purely out of fear and nothing else. She realises that having more people to select from will mean there is less chance of herself being selected for sacrifice by her own friends and family. When we are faced with the prospect of death, fear can make us turn against anybody else, even our own flesh and blood.