ANSWER:

Yes, this definitely falls under backbiting which is a major sin. She has no right whatsoever to want her daughter in law to serve her! She can keep a servant for that!
As a daughter, you must try and make her understand the Islamic ruling regarding this topic. But if she is as described, this means that she is mentally disturbed and her daughter can keep silent to avoid being in such a condition.

Mother and daughter in law living in separate cities. Mother unhappy and wants her to serve. mother tells her daughters about the problems and complains (truth/false) she has from her daughter in law. If try to turn the talk, she cries and goes into depression and becomes very angry, hyper and abnormal. Does her complains(true/false) fall under ghibah or slander? What should the daughter do then?