I'll probably get jumped on for saying this, but there is another possibility that happens in real life. I'm going to speak only to a hypothetical relationship and since we only know a tiny bit of the extent (for now) it's all any of us can do.
Husband was (or still is) attracted to and acts on inappropriately young females. Couple has several female children.
There are women who are jealous of the attention the husband gives their children. It might be just insecurity or narcissism that has nothing to do with sexual attraction or incest, just the attention and time that the wife thinks should be going to her alone.
It might be sexual jealousy whether it's warranted or not. Wife punishes children.
It might be sexual jealousy because there's specific knowledge. Wife punishes/victimizes children.
Wife punishes the "rivals for her husband's affections" (again) by withholding not only her affection, but basics of life like food, water, hygiene and body functions and access to outside world including relatives.
Bottom line: could be that yes, DT sexually abuses the children and LT is jealous and punishes the children because of it. She'd rather continue to provide him with his "needs" by allowing the abuse and continuing to provide more victims by having more children than lose him or the illusion of Happy Families. It's beyond sick and twisted, but it occurs it real life. LT knows what husband does and DT knows what wife does. Each has their role and is fully aware of and complicit in their spouses behavior and it's a mutually beneficial relationship as long as each gets what they wants. One may want access to sex with children and the other needs the facade of a perfect family, repeated wedding ceremonies to prove to the world how in much in love they still are...
I may not word this clearly, but I'm pretty sure it's understood. Understood intellectually, but not acceptable and yes, jeezopeez it's horrendous like a horror movie in real life.
I think of the paul bernardo / carla homolka relationship. SICK freaks.
Also, joel steinberg / hedda nussbaum - similar, but different, but fits into the sick couples group.
Just a note: I know firsthand (NOT MY RELATIONSHIP - yikes!!!) that there are husbands who are jealous of the attention their wives pay to their children, but I'm pretty sure that's not the case here.
Again, it's just an idea, I'm not saying it's so.