... about everything?
I mean the TV show implied she was blissfully unaware as Leland kept drugging her, and she never really showed any fear of Leland. But in the film she looks a lot more concerned, and even hesitant to drink Leland's concoction.
From what we're shown in the series and movie, Sarah Palmer must have occupied some part of the gray zone between utter ignorance and complete knowledge. At the end of ep2 after all, she's the one who goes to it, "What is going on in this house?!" and underlines it again as the funeral broke up next day.
I think she might have caught something once or twice, but not recently, like her "vision" --recollection?-- of Bob at the foot of Laura's bed or maybe something with Maddy at Pearl Lake that caused a rupture in the family. Whatever it was eventually got papered over so she could almost pretend it was over. Of course, based on her many anxious behaviors it's hard to think that she truly believed this. (That last glass of milk was practically forced on her, wasn't it?)
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"The bonsai: the ultimate miniature."
--Will Hayward, Twin Peaks
In my opinion Sarah didn't know at all. Feeling something is amiss isn't the same as knowing your husband is raping your daughter.
I think in episode 17 of Twin Peaks Cooper spells it out for us . . . he tells Mrs Palmer that her husband drugged her to conceal his actions.
Her hesitation to drink his "concoction" might have been for a number of reasons (e.g., perhaps she knew the hallucinations that would follow and found them terrifying)
OK.
Just notice how much of the study of denial in Twin Peaks is displaced away from Sarah Palmer. It would be cruel to be too specific. And inaccurate, because it's not just her problem, it's the one thing where she actually gets social support.
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"I am The Arm."
--Twin Peaks: Fire Walk With Me (1992)