I think Sarah knew, but was in denial. She is presented as slightly mentally disturbed, which could have come from years of guilt and fear, living with Leland. She didn't have the strength to face the truth, so she buried it and kept on living.

This is one of the things where the supernatural elements of TP don't interfere with the reality of a violent father in a family. The way Sarah behaves is quite realistic.

--
Vesa 'wtw' Ahola wtw@iki.fi.REMOVE
If you chose to dance with a bear, don't stop when you get tired.

Well I don't think wether or not she has seen BOB in Leland matters all that much - I don't think Leland physically became BOB when he'd go to Laura, she just saw him that way, if not because of a certain paranormal ability, then because it was easier to accept, especially since she was only 12 when it started...
Surely Sarah notices being drugged and the fan being turned on at the most ridiculous times? I'll go with her being pretty much an emotional wreck already, and denial, or looking the other way. I mean, wouldn't he most likely be the reason for the mental breakdown she's suffered?

Sarah was in denial, but I think that obviously she *knew* something. Maybe because of her gifted nature, but not only because of that; something terrible is going on in her own house for 5 years, he married a man already possessed by an evil entity, and there are the drugs, the fan, the mood changes in Leland, etc., but she denies herself all that, focusing in how much the three love each other either, her simple life (what does Sarah do other than smoke, read or going to church?)...always in a subconscious level...

After Leland dies she forgives him, yes (in a great scene with Coop), but because she understands her husband was not to blame, but BOB...

An interesting subject of discussion is what has been mentioned: why Sarah begins seeing BOB?

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?

Actually, Cooper talks to Sarah and "the Leland" plenty of times. Which show are you watching? 

"watch out for my cousin.."

Which episode was it?

MAYBE (and i am going out on a thin limb here)
BOB became manifest to sarah in order to set certain chains of events in place
i.e. the death of leland, his inhabited, and then the fall and inhabiting of cooper at the very end
what does earle say in the videotape the major shows truman and coop?
how the dugpas(which is actually a tibetan buddist term, i think it means 'the yellow hats')
anyway how the dugpas propitgate evil for evils sake, everything they do is to manufacture and then feed on garmonbozia(pain and suffering) and if they exist outside the realm of time, BOB would already know coop to be his next vessel. like bowie says in FWWM, as he points to cooper, "just who do you think that is there?"
appearing to sarah only hours after laura has been killed is not only a psychic clue, but also the setting of the trap for cooper.

just watched the show again, but must've missed this one.

what are 'dugpas'? when are they mentioned in the show?

my user name is spelled incorrectly.

tatemae 

And how do you guys know that? I could never find anything about them