My mom is such a horrible liar...she lies to my sister which she lives with, terrible lies, manipulative

She learned to do that as a coping mechanism towards my stubborn dad, to turn things her way...
I don't argue with that any longer but I feel sorry every time we talk and she tells me about her lies, asking me not to tell anyone. I can't bother telling her that I don't want to hear these stories any longer, I've argued for long about these and now I only let her be. I have to accept her the way she is and unfortunately knowing that this blocks my sister forever for healing, as she's getting further traumatized. Aah.