The revelation that Quentin and Judith always hated each other and the games they play make one wonder how they grew up.
Interesting touch in that scene with Quentin and Judith which I didn't see anyone mention. He's all his normal cynical exterior in that scene -- until she says that line about him hating her. At that, he seems to drop the defenses, and says quietly "I thought you always did me,"and asks rather sadly "Why didn't we get along?". Nice touch showing the vulnerability underneath the bratty bullying exterior. The man behind the curtain isn't as tough and powerful as he desperately wants the world to think he is.
And I agree with ER -- I DEFINTELY peg him as the baby of the family, Carl's behavior notwithstanding. That also explains a lot of the family's resentments toward him, esp with Grandmamamama playing them all off against each other. He's the baby, he's very bright, can charm birds out of the trees, and is drop dead gorgeous. Judith and Edward are the "good children" all duty and responsibility and not much sense of humor. Carl probably started his tricks to get any attention at all after being supplanted by "the baby: and has been overshadowed by him ever since.
The best analogy I can think of with Quentin is Edward VII, the son of Queen Victoria. Same sort of philanderer and scalliwag, had no useful occupation, his mother wouldn't trust him with any responsibility, so from sheer boredom he just played his life away waiting for her to die so he would have something to do. Interesting thing is, given responsibility and trust for the first time in his life, he turned into a rather competant king.
As to "ruining" Jamison, hard to say if there's reality there, or just the sense, common among Victorians, that not working all the time, thinking of nothing but duty and responsibility; wanting to play and have their own interests would "ruin" the child in the long run.
Jeannie