They need to adopt the Wandsworth Council model. Siblings only have a priority up to a certain distance. Eg: 1 mile. After that you no longer have priority (Wandsworth is 800m).

Personally once someone is in school and settled I'd rather they stayed there, and it is pretty hard to get 2 children to different schools for the same start I imagine - I think it could only work if you moved both children to whatever new school they were allocated.

I can see why they did it, but in practical terms I'm not convinced it's the best plan. I don't think there is a great answer to this, but personally I prefer the status quo.

Take care about school catchment areas. I know someone living in Westwood Park (Tewkesbury Lodge Estate - up on the hill) in a house no more than 400 metres away from Horniman Primary who didn't get a place there last September. They were only offered a place in a school in the middle of the estate at the Nunhead end of Brenchley Gardens.