I think that sarah new what mama was doing, there was so much evidence, like there wasn't a doctor in Peachester, and mama not letting the kids play near her garden, and kids getting deathly ill overnight! I think she knew but didn't want to believe it so she just blocked that option from her memory. Do you think that Sarah was afraid she'd be next? or was mama so reliant on her that she couldn't kill her? Couldn't she have just gotten the second oldest girl to help her out?