"Missing (Fear Street #4)" by R. L. Stine

Mark and Cara Burroughs have been home alone before and don’t think anything of it when their parents don’t come home one night. But as time passes with no word, the siblings start to worry. Where could they be? And why don’t the police seem interested in tracking their parents down?

Strange occurrences start to pile up as Mark’s girlfriend vanishes and Mark and Cara’s strange cousin seems to be watching their every move. But the true terror is only beginning. Someone wants Mark and Cara to disappear, too! But why? The answer lies deep in the Fear Street woods—if they live long enough to find it.