The Sign Of The Ram by Margaret Ferguson

Sherida Binyon soon learns that her opinion of the wild Cornish coast to which she has come applies to more than the scenery.

For at Bastions, the strange old house in which she works and lives, she finds another sort of cruelty--a menacing human kind that delights in power and would kill to keep it.

Sherida knows she is in deadly peril. Many hostile persons live at Bastions, but she realizes her life depends on escaping the sinister schemes of one born under THE SIGN OF THE RAM.

Sherida comes as companion-secretary to crippled Leah, passionately adored by her whole family including sixteen-year-old Christine. Subtly playing on Christine’s emotions, Leah spurs her to the point where she attempts to murder Sherida. On the surface, the motivation is simply the love of power, but Christine’s emotions are clearly variant; when the book was filmed, they carefully cast Christine as a girl of eleven, to make it unmistakable that her adoration was only “childish. ~ Marion Zimmer Bradley