Empathising with an abuser doesn't mean that one agrees with the abuse. I can understand what drives "Okaa-san", but I don't think treating her daughter the way she has done is the right way to handle the situation. Taking your pain out on somebody else who is undeserving of it is easy, but doesn't resolve anything.
The fact that readers have been throwing hate at the characters and plot are, in my opinion, a sign that the characters and plot have had a strong emotional impact, which (again, in my opinion) is a sign of good writing. I just wish more people were able to express that a bit better...