In the Shadow of the Tower — Ruth McCarthy Sears

Gina had never liked her stepsister, Merilee. But she could not afford to refuse the offer to come to San Francisco to care for Merilee's young son.

So with an old and ill family retainer, she sets off for the luxurious Nob Hill mansion where her stepsister reigns with her darkly handsome husband, and arrives only to find that her young charge had been drowned and Merilee was mysteriously dead--or was she?

Gina found herself in an inextricable web of conflicting and puzzling passions--and her own death seemed to be the only solution!