I've always felt the right way to approach this is to 'marry' a parent with a child in your preferred school.

Many schools when they have a bulge class will include pupils from more than 800m away, so to suggest that sibblings would not have priority when the first place was achieved through standard measures of distance, seems unfair, and to suggest this rule would only apply if the family move (possibly to a house slightly closer than the distance when they got their place, but still beyond 800m).