MK11+ wrote:
Their admissions policy states that siblings are priority number 4, but doesn't state whether that applies to in catchment or outr of catchment. Looking at the distances, it would imply that it is only for in catchment children. Would appreciate any one with insight to this process as we are thinking of moving house on the back of the conclusion to this!

The 2018 policy gives siblings (wherever they live) priority over catchment children. However, if memory serves me well, the 2019 policy reversed that, with catchment children taking priority over siblings. I read the consultation document, and I thought the 2019 policy was now online, but it seems to have vanished.
However, given that SHF will never fill all its places from catchment, it is irrelevant. Siblings have priority over distance-only allocations, which will be the bulk of the admissions.
You need to wait for the 2019 policy to be published before moving house!