How do I tell my family to backoff without causing more problems?

My Mother and two sisters are not very fond of my wife, or so she says. In fact she tells me that whenever I'm not around they make certain comments and they do not respect her. My wife is very upset and she wants them to accept her as I have accepted her. If I go to them and tell them straightforward to stop being bitchy towards my wife things might just get worse. How can I help stop this? What should I tell my family? What can I tell my wife or do for my wife to make her feel better about it all?