What do you think is the Council's motivation for doing this? As a long term measure, presumably it's a move towards a 'distance from school' approach, perhaps then towards abandoning catchment areas? (I think the City Council operates more in this way).
Is your concern solely about the speed and lack of transparency with which this is being implemented? So your aim is to gain a stay on implementation, until children currently in schools have left?
I understand your upset at the lack of consultation and publicity. Not knowing (being unable to know) what criteria would be applied to your application is clearly unacceptable.
I don't quite understand your comment that 'the new rules are totally against family values' which appears to be about the policy in the longer term. Is that what you mean? Surely once people know what the new rules are they will make application decisions accordingly. So, people will be more focused on distance to school for their first child, if they wish subsequent children to get into the same school. Or they'll play safe and choose their catchment school.
There are lots of cases at the moment where a child gets allocated an out of catchment school, then a sibling doesn't get offered the same school because it has become more popular, so only in-catchment children get in that year. So the 'children in different schools' scenario isn't unusual. I can see your issue where you were confident the school would not be oversubscribed though.
If the council is thinking about dropping catchment areas things would become more 'interesting', as there could be no expectation about siblings gaining places. (Though as RL12 points out, this is not guaranteed at present - presumably where there are so many in-catchment siblings that those further away do not gain places, and no first children in catchment do, is that what happened?).
Anyway, lack of transparency is not good.