The fact that two children were abducted reminds me of the case in Australia in (I think) the 70s. It happened at a huge game -- football or soccer or some such -- in a large public arena. The two girls didn't know each other, but their families were sitting near one another. One girl was around 4, the other around 10. The ten year old was asked to take the younger girl to the bathroom. They never returned. People questioned how two children, one of them old enough to scream and make noise, could be abducted in such a busy place. Recently a man has come forward saying he saw a little girl around 10 angrily chasing and shouting after a man who was carrying a smaller girl. He thought it was a domestic disturbance at the time; now they think it was the older girl trying to save the younger girl.
If Scott Fandel was trying to save or protect his sister, he might go along willingly with an abductor. Also, it could very likely be someone they knew, even if it wasn't a family member. Didn't it look as if they had been cooking something on the stove? Maybe someone came in, said, "Your mom's hurt" (or something to that effect), and gotten both to go willingly along.