Mother knew about sexual abuse

I spent the first 9 years of my life with my parents living together and my mother being treated like dirt. My earliest memory is of my father standing at the sink playing with his genitals while I was in the bathtub. His behavior towards me was what I count as being sexual abuse, even though there was hardly any physical contact. Now it's years later, my father is ill, and I am still hurt and angry. I have been suicidal and am in counseling. My brother and sister both love our father dearly. I confided in my sister, and she didn't believe me. I never told my mother, because she was always so close to a breakdown, and is still struggling to get her own life together. Lately, as I reflect on confronting my father, I wonder if my mother should be told. After a lot of thinking, I still can't decide which would be worse - if she didn't know and doesn't believe me, or if she did know and did nothing.

Could a mother live in a house where her daughter was being sexually abused for six years and not know about it? Why would she, once I grew up, cut my father out of my life so completely that he wasn't invited to my wedding? Could she honestly not know? Could she not have figured it out? After 20 years will she believe me, and if she does, will it push her over the edge?