The Deadly Sister by Eliot Schrefer
Abby has always covered for her troubled younger sister's (Maya) misdeeds. But now, when Abby finds Maya's boyfriend dead in the woods with Maya's cell phone nearby, she is going to have her hands full because it sure looks like Maya killed Jefferson. Abby begins playing detective to see if she can find a more likely culprit. It turns out that Jefferson wasn't the angel everyone had perceived him to be, and there are plenty of people with motive to kill him.