One problem with the sibling policy in over-subscribed schools is that it enables a family to get the first child in on distance (having possibly rented a place to apply from), and then move away and all the other siblings get in, meanwhile every year a new family gets their first child in and then moves away...and the accumulative effect is that lodas of siblings get in and the actual distance catchment for first children gets smaller and smaller.
I can see why some families would find it much easier to have children in the same school especially in rural areas where the schools might be miles and miles apart and very little public transport.
And why it might be important for some to send their children to the same school, but in terms of justification as to why a child has a priority over others to get in it seems a bit unfair. i.e a child who llives within 800m of the school really wold seem to have more of a case than a yr7 sibling of a child in their last year, who lives 4 miles away.
In any case, the children have already been in separate schools for years while the younger sbling was still in primary!