Cory is occasionally inferred to be quite sickly and is the first (and only) to die from the arsenic. Studies have shown that inbreeding can lead to weaker immune systems, allowing for more difficulty with illness. Chris and Cathy have the advantage of being older, and one can only assume that Carrie was the healthier twin. With that in mind, it's not surprising that Cory didn't last long.
While Chris and Cathy's relationship is questionable (due to the Westermarck effect), there is some evidence for their parents' attraction to one another. Siblings, and even parents and children, separated in early childhood and reunited—in some cases, unknowingly—later in life have reported a powerful (though not always sexual) attraction to one another. There is a theory that there may be an instinct to recognize persons with whom one shares a close genetic bond, and that lacking other social contexts, this instinct might be interpreted by the brain as a romantic or sexual attraction. Corrine and Chris Sr. were separated when Chris was three and Corrine was an infant, only to be reunited when both were in their teens. Although they believed themselves to be uncle and niece, they may have actually recognized their closer kinship but misinterpreted the attraction as Love at First Sight.
As punishment for her incestuous marriage, Corrine receives forty-eight deep whip cuts, going all the way down to her ankles. Yet, very soon after, Cathy describes her playing tennis, and wearing shorts and backless dresses. Not only does Corrine show no lasting injury (in Real Life, that many lashes can cut flesh from bone), but no one outside the family seems to notice the hideous, crippling scars that should have come from such a whipping?
As noted on the headscratchers page, she could have used makeup or the lacerations weren't deep. However, it's not said what kind of whip Olivia used, in which case, she could have lashed with some kind of switch, something that could leave a cut but not bad enough as to where it would leave behind scars.
A lot of the drama goes out of the book when you remind yourself that these kids have had a rope ladder since the very first night. Almost as soon as they're locked in, Cathy begins to worry about what would happen in the event of a fire, and she and Chris make a Bedsheet Ladder to stash under the bed. They then spend the rest of the book coming up with excuses not to use the ladder. At their most desperate moment, during which the children are being starved and Chris is reduced to feeding the twins his blood to survive, the excuse is that Cathy and Chris are too weak to climb down the ladder with the twins on their back. This doesn't explain why either Cathy or Chris doesn't escape alone to summon help for the others. The fact that immediately after this scene, Cathy and Chris use the ladder to escape and go skinny-dipping makes it even more What the Hell, Hero?
Here's the thing about abuse. Cathy and Chris were still holding onto hope that their mother would come for them. Sixteen years of affection doesn't just magically go away. And from their perspective, their situation isn't ideal but it could be worse. They have beds to sleep in and regular food - and the attic is a decent playing spot. If they escape, where do they go? They're two teenagers with no money and a pair of young children to also take care of. It's not until they realise that Corey was poisoned do they leave - because they know their lives are in danger. Yeah they could always have escaped, but they were at first hoping their grandfather would die soon and they'd get the inheritance. It's not getting out of the house that's the problem - it's what they do and where they go afterwards. When they do decide to escape, they make sure to steal plenty of things so they'll have money.