All in the Family
The world of Harry Potter is filled with ancestry and alliance, clique and clan, blood and brethren. Once we have met a character in the first book, we can be almost certain another of their bloodline has popped up somewhere between Harry’s 11th and 15th birthdays. The only central character that we know of to have no siblings and a relatively normal life is Hermione.
Hermione is clever but determined, and she reacts to refusal with the intellectual equivalent of stamping her foot. No is not an answer Hermione Granger abides well and, despite their best efforts, Harry and Ron usually find themselves drifting toward Hermiones way of doing things.
Some siblings are very alike. For example, I personally cant tell one Creevey from the other. They were sorted into the same house and somehow both managed to end up with a rather unhealthy–though highly amusing–amount of Harry-worship. One might say it is fair to assume that they grew up in a Pro-Potter (rather than a Vote Voldemort) household.
The Patil twins (although Parvati was sorted into Gryffindor and Padma into Ravenclaw) both seem quite similar to me– beautiful, proud and haughty.
Though one must wonder about the generations before them.
Sirius and Regulus: one a filthy blood-traitor, one a proud Death Eater and apple of their mother’s eye. It seems that, from the beginning, the two were destined for different paths. Sirius (as we know) means dog star, whereas Regulus means prince. The Prince and the Dog. The hither and thither of paths as far as life in an ancient pureblood family goes. Sirius himself said, Of course, anytime the family produced someone halfway decent they were disowned. We know that Tonks mother Andromeda was stricken from the family tapestry for daring to marry a Muggle, whereas her most charming sisters, Bellatrix and Narcissa, made fine, respectable marriages to respective future death eaters (what proud pureblood girl would accept anything less?). Bellatrix killed Sirius without pause, while we know that her sister Andromeda was his favorite cousin.
So we see that differences do show amongst siblings of the older generations, but what of the new?
Take Percy Weasley for example. The Weasley family is very close-knit. Though they squabble, one cannot help but notice the love between the siblings. One would never imagine that a Weasley of all people would be one to betray his family for want of power. Then along came Percy Weasley… From the first, we knew he was different. Proud, well-spoken Percy: so different from his funny, sarcastic brothers. He seems to be the only one in the family who resembles none of the other siblings (aside from Ginny, but shes a girl–shed be unfortunate indeed if she resembled her brothers too closely). Charlie, we are told, is stocky and short like the twins, while Bill is described as taller and lean like Ron. But Percy we see merely as Percy, as different in looks as he is in character. Percy, who stays locked in his room while Bill and Charlie make tables fight in the yard. Percy, who haughtily demands quiet in the halls, writing a paper on cauldron bottom regulation while explosions can be heard from Fred and Georges room.
J.K. Rowling seems to be pressing the idea that people are born a certain way: that good choices can be made by good people, regardless of what they are taught, and anything less is unacceptable; that goodness and badness need not be taught, but are, rather, inherent traits; bad people choose to accept the bad ways of their families, but there are those who are not afraid to break away from the fold and choose the way of good.