If you haven't already, and if you are comfortable doing so, you could try telling her that you feel hurt and uncomfortable when she says bad things about your dad or family. That might be more effective than telling her she is committing a sin.

Usually, it is considered irresponsible for a father or mother to say bad things about each other to the children (unless it is an urgent or important issue) because it is psychologically and emotionally harmful and makes the children feel a conflict of loyalty between them.