Questions about a Passage

Well to be totally honest, when I first started reading this book I didn't really understand it, let alone enjoy it. But after rereading some of the chapters and going over them on Spark Notes I am understanding it more and more. But the one thing that I am still not quite getting is the fact that Mrs.Joe is so mean to her little brother. If I was in her place, I would want to be a thousand times nicer to him because he has no one else but me. I don't know if the author is just trying to be dramatic, or if there is some secret motive around her being this evil. I feel for Pip because of the situation he is in, and how uncaring his guardian is. I guess that the times are different and Pip and Mrs.Joe's relationship is weird, but if both my parents died, my siblings would welcome me with open arms and would never treat me the way Pip is being treated. Thanks for your help, and I am ready to fully understand what the author is meaning by this.