I think that the killer must be local, within 10 -15 miles, close enough to be very familiar with the area, terrain, roads, etc.
What about someone who knows the older sister who dropped them off? Possibly she was talking on her phone, or to a friend, and someone overheard her saying, "I've got to run home and pick up Libby and Abby because I promised to drop them off so they could hike the bridge."
i keep thinking about other cases of murder or kidnap where the perp turns out to be a family acquaintance, or friend-of-a-friend, or neighbor, or former boyfriend of an older sister, or someone else who was familiar with the family and the family habits and activities.
If I were LE I would be checking out any former boyfriends or friends of older siblings, the newspaper deliver guy, neighborhood handyman, etc.
When I lived in another state a criminal turned out to be a substitute newspaper delivery guy. He subbed for the regular delivery guy who was on a three week vacation. The sub pulled up to over 200 houses to deliver the daily newspaper and he observed who was home at that time of day, saw kids getting off school buses, etc.
I know I am rambling on here but just wanted to put down some of my thoughts.