I don't think that it is the Uncle, but he obviously knows something, like who it is. Well, we know that that uncle has at least 2 daughters in their early to mid 20s, from an interview they gave to O'Reilly. I wonder if that uncle has sons around that age. Would explain a few things...
I have been thinking that it could be a cousin that was having a relationship with the girl and that the family was unhappy about the closeness of the relationship, so they staged the abduction. It's the only thing that makes sense to me, that could explain why the girl didn't scream like heck, instead of going willingly, as well as the pollyanna optimism of the uncle and his family.
Did you catch the way the girls even evaded answering the question of what they would like the public to do. They said to just keep the love and prayers coming their way and as an afterthought, they said and to keep their eyes open of course.