I really think, considering the mother's medical issues, that she was probably not strong or coordinated enough to do anything to that child. Far more than I would see the SF's medical issued being something that could keep him from being able to pull this off. It is possible that the reason that the mother has not said anything is because she is embarrassed and terrified of what all is going on. Not just her daughter's disappearance, but the TV cameras and all these strange people being around. If the area is as rural as I am thinking (having never been there) then all of these new people being around and the thought of them putting her on TV nationwide might be more than she can deal with, especially under the circumstances.
Like I said, I have never been there, but when I was volunteering with the IP, we worked a case that caused me to have to go out to a place in VA that was very rural in the mountains, about 12 years ago. I literally saw someone in the front yard of a very run down house taking a bath in one of those big round metal tubs/buckets, with the handles on the sides like you see people taking baths in in cartoons of rural areas. I couldn't believe it. I left with a new appreciation for my life. When we pulled up to each place we had to go, to ask people questions we had to be careful how we acted and how fast we were with everything because as strangers the people did not trust us and they seemed kind of scared of us. If we were too quick or too intrusive and didn't ease into things you could see them getting nervous and you had to talk them back down to where you were, or they sometimes even said that they didn't like the situation and would clam up and ask us to leave. Just a thought. Not at all saying that any of these people are like something out of Deliverance. Just that TV is intimidating for lots of people, even when they aren't in rural areas and are used to seeing TV crews in trucks on their street. When you live a life sheltered enough it might just be too scary. If she is ill, she may be extra self conscience. I'm just not reading much to the mother's lack of being on TV, yet. Like I said, I have never been there, so I don't truly know what it is like there, but I could see that being the case where I did go.