Circumstances such as hoppinggreens are different to those who deliberately play the system.
The sibling rule is needed as theres no way to get children to two schools that start at the same time. Yes there are breakfast and afterschool clubs but they are expensive and why should parents have to pay £90 or more every week because their dcs didn't get into the same school?
Yet at the same time it's unfair when parents play the system at the expense of children who then have to travel to another school as their closest is full of siblings. Something needs to be done such as putting out of catchment siblings that have moved further from the school behind those in proximity. In circumstances such as hoppinggreens it would be easy to see that they didn't move once the first child got a school place so they should be counted with the within catchment siblings.
I live on a huge estate with four primary schools (though one is a faith schools) within roughly half a mile of my house all starting at the same time each morning and finishing at the same time. There's no way I could get dcs to two of them every day as I don't drive and no way can I afford breakfast and afterschool club, if not for the sibling priority there's a real chance that dd2 wouldn't get into the same school as dd1 and ds.