We don´t know anything about the dynamics between the siblings.
There may be a reason to split them up.
Often abusers like the Turpins have their favourite children who help with the abuse.
I am not blaming anyone (but the parents), but think about the Knorr case.
Theresa Knorr groomed her boys to torment the girls.
There have been other cases, can´t remember the names from the top of my head.
So we just don´t know.
_______________________
You have to wonder WHY this happened.
There is a lot of hate in what the parents did.
Louise Turpin was sexually abused (grandfather?) when she was a child.
That would have messed her up big time.
What about David Turpin?
To me he looks like he could have mild to medium autism - has anyone else thought that?
Was he easily led?
Who was the "leader" in all this? Louise or David? Both - a united front against their children?
In the case of the young man, Wesley, that I linked earlier, (and again, below) his siblings had tried to sneak him food. They got caught and then were afraid to help him anymore. He was his step-father's whipping boy. His mom stood by and did nothing. She knew, before they married, that he hated Wesley, she'd witnessed him punch him in the face, before they married.
It only took a year for the starving to cause a very significant change to appear in Wesley (along with the severe physical abuse).
"I didn't recognize who he was," says one son, Scott Davis, 16. "Because I had gotten a lot bigger, and he had gotten a lot smaller." They invited Wesley to sit down and have something to eat. "The little guy, he was starving to death," Smith says. "He was eating everything he could get a hold of. I mean, he was just gobbling it down."
This is why I think that there may have been periods of the T sibs getting more food. I don't think anyone could survive on a meager meal, once a day, from the age of a very young child, to adulthood. It would lend to why they had growth spurts, allowing them to reach heights as tall, and taller than, their mom, and in some photos, some of them looked to carry a bit more weight,than others, too. If one or more got caught planning an escape, or attempted one, or broke a rule, one or all, may have gotten put on lock-down, and on the one meal plan. Perhaps for weeks/months, with the most serious offender(s) shackled, as well (three at the time LE came to the door b/c they'd discovered an escape attempt, iirc.).
I think the parents had come up with their own stripe of "religion" but like other abusers, may have used it as their justification to do what they did. Just using as an example, the Duggar girl spoke of being lifted by the throat for not brushing her teeth.
There are many who are sexually abused, and who have had trusted family, do highly inappropriate things to them, that they struggle with throughout life, but it comes out in different ways for different people. LT may have started out being fearful of letting them out into the world, and her paranoia grew into something very sick, and twisted. I see her as having some sort of psychiatric problem and, DT seems to be very awkward/backward, but I'd not make the mistake of thinking of him as being a pushover.
IANA psychiatrist, just going on the videos, and gut, FWIW, I think the two became a united front. They remind me of people who are very wrapped up in each other, and "things", but the kids, not so much. Had they married someone else, this may never have happened. Together, it was a match and gasoline.
Wesley's story:
http://articles.latimes.com/1997/jul/27/news/mn-16711