Actually, in intact families, most parents no longer spend the same amount of disposable funds on the firstborn after the birth of subsequent children.
Just as a first born should not "suffer" because of the birth of siblings, neither should a latter born child, who themselves have no say in their birth order, be treated as far less entitled to support from their parent. In intact families, children recieve roughly equivilant support by parents - who do adjust spending on earlier children as they provide for later ones. Not Most to the first, and little to the latter-born.
Heck, I'm a first born and darn tootin my parents spent less on me when my sibs came along.