There's not much to our supporting actress, Sally. She basically just hangs around and echoes what the narrator thinks and does. How very, very boring.
Examples? Of course.
When the narrator starts getting nervous about the Cat in the Hat showing up, Sally chimes in to say that the Cat in the Hat is no good. And when the Cat in the Hat messes up their mother's dress and the narrator freaks out, she's totally on board:
But that dress! What a spot!
"It may never come off!"
Sally said. "It may not!" (86-88)
Sally is probably just a projection of the narrator's anxieties about the Cat in the Hat. Crowd mentality, perhaps? That might fit into the whole Red Scare allegory, making Sally one of the people who bought into the communist paranoia back in the day. What do you think?