This is an excellent question, because we only ever find out the cause of Charlie Marie's depression at the very end of the story, when it transpires that when her first husband left her, he took with him Tex and Belinda, their two children, and that Charlie Marie only ever saw these two children once again. This helps us understand the incredibly erratic and chaotic lifestyle and behaviour of Charlie Marie in the novel. Her large number of marriages, her dependency on others and her depression and anger are all a result of having "lost" her first two children suddenly and never seeing them again.
It is of course a narrative strategy to withhold this information until the end of the story, as it forces us to wonder along with Mary and her siblings as to the reason for Charlie Marie's behaviour. Such a technique forces us to play the same kind of guessing games and speculate just as Mary and her siblings have to before we find out the truth.