It's not quite the same situation, but a friend of my parents' sexually abused me when I was a child. I told my mother when I was 18 and the way she reacted told me straight away that she knew all along. She wasn't shocked, at all, and she didn't ask who it was, she just knew. Looking back I realised that she was actually complicit in the abuse, not in the sense that she wanted me to be abused but in the sense that she should have known that a man wanting to bathe a young child (against my will, may I add) was suspicious and yet she did nothing to stop it, in fact she insisted I let him (to save her the hassle, that's all I ever was to her, hassle). When I told her about it she told me I should just get over it, that I was not to tell my dad (who I am positive knows nothing about it) and that I was trying to make her feel guilty.
For years I made excuses for her. She had a very tough childhood, and had to work her fingers to the bone when we were young as my dad was unemployed and a bit feckless. But, having been honest with myself about the whole thing, I now realise I blame her for the abuse far more than I blame the actual abuser. I am far angrier at her than I am at him. It was her job to protect me, and if she failed in that, which anyone can, then it was her job to help me come to terms with it. In both situations she just didn't bother.
I am still in contact with her but as she lives in a different country I don't see her very often. Moving away from her was the best thing I ever did. I spent my entire childhood and right up to my early twenties trying to make her care about me, but finally I realised that all she cares about is herself. Strangely, I would also call her childlike, in the same sense that another poster called her mother childlike. It's like she has the practical capability of an adult (holds down a very responsible job, can run a household extremely well) but the emotional intelligence of a nine or ten year old. As other posters have said, my anger towards her has only increased since I had my DS and realised the love that a true mother feels.
She isn't my mother, not really. If she died tomorrow I don't think I would feel anything other than regret that I didn't tell her exactly what I thought of her. I hold back from that for the sake of my wonderful little sister who still lives at home.
Once my sister leaves home I will still see my mother a couple of times a year so that DS can have some sort of relationship with her. She is a good grandmother. But if she gets ill or needs help there is no way I will do anything for her, she can rot.