I understand fully your reluctance and what she did will not be justifiable in any circumstances, however occasionally good people make appalling judgements, often because an abuser also trades on their insecurities and weaknesses and finds a way of twisting and controlling them. Often these are the very weakest and most vulnerable people who cave into that sort of pressure. I don't know your mother, I know she made an abysmal choice in covering for her partner but that is not to whole of who she has been to you. What sort of mother was she previously, and are you absolutely 100% certian that she definatly knew at the time and not simply peiced it together afterwards? I would see her if at all possible if only to get closure for yourself, clarity about things which may still be troubling you and so that you can rebuild your life with a clear conscience that you did everything possible to make a truly appalling situaiton as right as it could be.