Describe the relationship between Isaiah and Charlie in "The Beans and Rice Chronicles."

Charlie is Isaiah's little sister. She is only four, and Isaiah is frequently asked to take care of her since his mother is often too depressed to get out of bed. Isaiah seems to care for his sister, and he takes care of her without much complaint. However, he does talk about how gross it is that she sucks her fingers. In addition, she can be argumentative, complaining for instance, that Isaiah does not cut her waffles the way their mother does it, and promising to tell on Isaiah for drinking straight from the orange juice carton.