i have direct experience of this issue - my mother married an abuser when i was 7.
i believe she stayed with the man who broke my bones and landed me in hospital for a number of reasons.
she was from a family of dysfunction, and often cycles of abuse simply continue.
she rejected me at birth and so had no bond with me, no mothering instinct. When my step father beat me, she did not get in his way. i believe that she was in her own way scared of him despite him never having laid a hand on her while i was there - however he did in later years beat her too, after both i and my sibling had left home.
I also believe that she was in denial. She had fashioned herself her very own happy ending, getting married after having a child out of wedlock (me) and to have faced up to the reality of my abuse would have shattered her own delusions of happiness.
she turned a blind eye. what other reason can there be? She went on to have my half sibling who also seemed to get treated very badly despite being my stepfathers birth son. I left at 15, and was homeless for a while. My half sibling met the same fate, but became an addict and lived rough for years. He is too messed up to have any relationship with - i tried recently. it didnt work.
i have no contact with my mother these days. i have not had contact for about 12 years. she never said sorry, never took any responsibility for her own actions in being complicit to abuse. She did try to contact me recently via facebook, but i ignored her message. My step father died a few years ago. (he was an angry little man and died young from a heart attack)
I on the other hand made a go of it, happily married for 21 years with 2 lovely kids - and if anyone ever laid hands on them, i would have done time, but then i believe i have a normal and healthy relationship with my two, and they are loved and wanted. I am not like my mother, though im not sure why. I can only attribute that to my very first years in the care of my grandparents prior to her meeting my step father, and my early years, while very unconventional, were loving until she removed me to live with her and her new partner at the age of 7. The only thing i have to thank her for is a very high pain threshold.
i have no idea how any mother can stand by and watch her child be abused. but sadly i come across it often now in my work. It always makes me laugh when the kids i come across tell me ive no idea what their life is like....
i am a police officer now. Funny how things pan out.