Sarah had a breakdown before losing Laura, it's obvious she was like that long before her death, the cigarettes, the reading [how to speak german?]...besides doesn't Laura mention in 'her diary' that her mum is afraid she's having a mental breakdown again at some point?
Sarah obviously [b]didn't[/b] see BOB 'in person' until after Laura's death, but I think that's maybe because she's coming closer to her senses, very slowly beginning to realize or accept the truth, although she doesn't really do so at any point until the truth is revealed by other means. Gum. Seriously, after Laura's death Leland is beginning to send out strange signs - BOB's face is more or less starting to show, the Tremond's grandson imitating the jumping man over Judy's - BOB is having problems there, same after Laura's death, there are forces working against him and it's breaking him down, maybe that's why Sarah sees his face...
So I guess she knows about him molesting her, and had she had a more stable mind, she would have known he'd killed her too?