Even if you hadn't read any further, the first sentence should alert the astute reader to the fact that the grandmother is a rather selfish character who doesn't realy think about anybody else except her own wishes or desires. The first sentence assumes that there are other people who do want to go to Florida, and the way that the story starts off with the character of the grandmother asserting her wishes in a sense creates a very strong image of a dominant female character who wants her choices to be adopted, no matter what other people think. This impression is supported just a few lines on, where the reader is told that the grandmother is ready in the car with her cat:
She didn't intend for the cat to be left alone in teh house for three days because he would miss her too much and she was afraid he might brush against one of the gas burners and accidentally asphyxiate himsef. Her son, Bailey, didn't like to arrive at a motel with a cat.
This is why the cat is hidden and she is deliberately deceiving her son, and going against his wishes, even though it is he who is driving and is the "man of the house." This supports the initial impression received by the reader that the grandmother is a very selfish character who is determined to have her way, whatever she needs to do in order to achieve it.