April Shadows by Virginia Andrews (#1)

The first in an enthralling two-part series. 

April had always felt like an outsider
Her older sister Brenda was tall, athletic, competitive, and sure of herself. But April Taylor was short, sensitive, and overweight -- and she couldn't bounce back from their father's cutting criticisms the way Brenda did. April didn't know why their once-loving dad had become a cold hearted monster, but she was sure it had something to do with her. And she could see how his cruel behavior was tearing away at her gentle mother. But a glimmer of happiness returns when Brenda brings home her college roommate: beautiful, bewitching Celia. And April wonders if she might not be so different from Brenda after all....