I first read SUMMER OF FEAR in high school and found it to be the first book I couldn't put down, actually reading it all the way through in one sitting! Lois Duncan takes typical teenage angst and self-conscious paranoia and spins them together in a web of horror and mystery.
Rachel is the number one priority of her family until her cousin Julia (parents recently deceased) moves in. The two bond as sisters at first, but when Julia begins to receive all the attention, Rachel's jealousy kicks into high gear. The spin here is when "accidents" and deaths become common place. Rachel suspects Julia to be responsible, but the jealousy Rachel's family has witnessed lend them to believe Rachel is just looking for attention, leaving her to discover her "cousin's" evil secrets and legacy.
A great suspense novel I still reread occasionally. Check out the movie starring a teenaged Linda Blair as "Rachel" and Fran (The Nanny) Drescher as her best friend!