Heather should learn how to count. We don't learn much about her during the movie, but we do see her flub the head-count when they're getting ready to go to the airport. She accidentally counts Mitch Murphy the neighbor kid's head, creating the false impression that Kevin is in the van.
Spoiler: he's not.
To be fair, it wasn't really Heather's fault—we blame Mitch for being a pest, and hanging around the van. But since Kevin never would've accessed his inner hero without them, we can congratulate them both. Good job—you helped your brother learn how to rig up a virtual torture chamber.