My DSis and I can't understand how my DM didn't see it either, WhatAShewOff, but her distress has certainly been genuine, we can see that. We both repressed the memories, though, until we had young DC, so maybe she's done the same. There's lots of things that we know she was aware of that she's denied all knowledge of.
She's good at denial and minimisation, though. She minimises the SA she suffered from her uncle. And there are things from our childhood that she admits to, which she minimised when we were little. She says that she knew that our F smacked us too hard and she didn't agree with it, but she didn't do anything to stop him.
I can see now that he was emotionally and financially abusive of her, and of us. He was very controlling and paranoid (he thought she cheated on him, the irony!). He was deeply misogynistic, he was convinced that a woman's adultery was worse than a man's adultery, though he could never say why.
He insisted that we moved to Saudi Arabia when we were small children, it was a miserable life for her. Thankfully, it was only a year. But it was definitely a pattern, that she was unable to stand up to him.
He also won the sympathy vote, as he developed Parkinson's Disease. Now, I can see that a man doesn't stop being an arsehole because he's ill. It made me feel really guilty after he died that I hated him, he made my skin crawl but I couldn't work out why. I've seen some of his letters to her when she was away, and I was shocked at just how emotionally abusive they were. She was so clearly liberated after his death.
So maybe the truth is, that whatever she might or might not have known, she was unable to do anything about it. It's hard to understand when you see how strong she's always appeared to be, but she was damaged.
I find it hard to be around her, though, so I'm very low contact with her. I can't imagine knowing so little about what's going in my DDs' lives.
Your experiences sound awful, too, I'm so sorry. 
