Helen and Betty are very good friends. They are in the same  class at school. Now they are eight years old. Helen's mother has a new baby. Helen is very happy to have a little sister, so she always talks about her to Betty. At first Betty is very interested in the new baby. But after some time she begins to get unhappy because Helen doesn't stop talking about it. One morning, the two girls meet in the library, and Helen says to Betty happily, "Do you know, Betty, my baby sister puts on  about half a pound  this week." "That is not much," says Betty. "I know a baby that puts on ten pounds a day." "Oh, that can't be true?" says Helen with a laugh . "Whose baby is it?" "An elephant's," says Betty. Betty and Helen are  _  .
sisters cousins classmates brothers