In the absence of any screenwriting insight today (help! I have to write a brother and sister bickering, and I'm an only!), here's a bit of wisdom from Scientific American
. A mathematician has proven that you can always balance a table on an uneven surface if you rotate it.
Unfortunately, it doesn't say what you do if the table's legs themselves are bent out of shape, as is the case in most Chinese restaurants I know.
In my experience (second of four kids), arguments between siblings, including adults, have two major differences from fights between, say, a married couple:
#1 - The argument is almost always about power, and not about achieving a certain goal.
Siblings fight to establish (or re-establish) the pecking order. The fact that Sister swiped Brother's book is less offensive to him than her "audacity" in thinking she could get away with it. The book itself may be destroyed in the fight, but getting it back intact is secondary to punishing her for taking it in the first place.
Victory is the goal, not harmony, and compromise is anathema. Unconditional surrender, followed by dire consequences, is the prize.
Birth order is a factor. Society says that "older is better," so the older sibling already has an acknowledged superiority. Keeping younger siblings in their proper places is a demonstration of that power. The younger ones can undermine this only by outsmarting the unsuspecting older sibling.
#2 - Siblings are much faster to go for the throat.
Sibs know each other's most vulnerable spots and have less incentive than a spouse to avoid them. If Brother knows Sister is sensitive about her "fat ass," that's the target he aims for the minute he's losing ground in the spat. (In fact, he was probably the one who told her she had a fat ass in the first place.) That siblings aren't usually dependent upon each other for sex, money or companionship makes whatever consequences might occur pretty harmless.
Also, the old cliche of "no one insults my brother except me," is true. Good luck and happy writing!
Siblings know each other better than most people, but actually understand each other less than they would an alien.
But take care to think of how far apart they were born. For every year in age difference, the relationship changes dramatically. (Not that that really helps you, or tells you anything you couldn't have already guessed).
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