This is not to make any excuses for Rebecca and Jack but I have always heard that three children are the most difficult to raise because one will always be neglected. If you have more than three, the children tend to pair up and help each other, leading to no one feeling like the odd one out.
I used to think that Jack spoiled Kate so much because he just was one of those fathers who thought their daughters were little princesses that could do no wrong. I am now beginning to think that it was more selfish then that. He needed Kate to need him. Heck, I wonder if he even enabled her weight gain to a small degree because that would mean he would need him more. He was insecure in his relationship with Rebecca, because he thought she was too good for him, and on the outside Kevin was usually so awesome that Jack did not feel needed at all by him, even though we know that was not the case.
My uncle was the favorite child of his parents, because he was the first born son. My mother and her other brothers got the short shaft and became very successful in life, though they never totally got over the insecurity that comes with not being the favorite one.
The favored uncle went on to have a stroke at a young age and died tragically. He could never handle anything stressful because he grew up extremely coddled. He was a total loser in life and never accomplished anything. He sponged off his parents and never made anything of his career (my grandfather used connections to get him into a prestigious field and school, something he never did for any of his other children).