No, it was her illness that made her a useful scapegoat.
Bridget, my understand of schizophrenics is that while they are on meds they are safe from the worst ravages of their illness. The minute they decide they no longer need meds can spell danger for themselves and others. When Sheila asked her doctor, a locum I believe, to adjust her meds, a chain of events with potentially tragic outcome was set in motion. Firstly, her meds should never have been halved, should not even have been reduced without strict monitoring. This appears not to have happened. Perhaps it was because Sheila wasn't within reach of her regular doctor, maybe she managed to avoid seeing any doctor or maybe her illness was tightening its' grip on her and she saw doctors as instruments of the Devil. For whatever reason, it seems she may have fallen through her safety net.
We know the farm wasn't a place of peace and harmony for her at the best of times, added to which, her boys didn't consider it their favourite place to be either and then during the drive, her estranged husband says he's met somebody else. Might she have felt that he was abandoning her. Hard enough for her to cope with on full meds, but expecting her to cope with all that stress without her full theraputic dose, would, IMO, be asking too much of her.
I think it can reasonably be assumed that by the time she reached the farm she was experiencing huge emotional turmoil which probably caused her illness to gain a tighter hold, which in turn, could have caused Sheila to loosen her grip on reality.