Often schizophrenics target their own families. They start seeing them as enimies who are out to hurt them (Sheila clearly thought her sons could harm her, even kill her!). In some cases it tragically ends in murder. 
Alias, if we take this a step further to take on board the moral code, regarding sex, with which June imbued Sheila, we may find that Sheila's greatest fear lay in that she MAY be induced -remember her fear that they would seduce her- to have sex with her own sons, the ultimate taboo.