http://transcripts.cnn.com/TRANSCRIPTS/1702/16/ptab.01.html
Tributaries comment from Sgt. Kim Riley in the above transcript. He also mentions dropoff and pickup info.
https://m.youtube.com/watch?v=_DiBYdeGbdE
At about the 9 minute mark a reporter asks Riley where the girls were found.
I've always thought it may be significant that they identify the person who dropped them off as Libby's sister but have not identified the person who was to pick them up except as a family member. I'm not saying I think a family member is directly involved, just that the way one was identified and one was not stood out to me.