The sibling rule is absolutely the right criteria for schools to adopt as to who gets in first.
Yes, it is frustrating that someone may have moved out of the area following the acceptance of a first child and the sibling has got in at the expense of your child, but as somebody has previously said, it would be wholly inappropriate from (amongst others) a logisitcs point of view to drop one child at school and then drop the sibling off at another school.
My first child was refused a place at her nearest primary school (a 10 minute walk) due to an oversubscription and the alternative school offered had an awful Ofsted report and was filled with the offsrping of chavs, so, (and I'm not suggesting this is the answer for you) we took the drastic action of moving to ensure our child got into a decent school.