That policy is very simple.
Say you have 60 places.
If there's a child with an EHCP naming the school, they get place #1.
Any children in care or adopted from care here or overseas get, say, places #2 - #5
That leaves 55 places.
IF there are 55 children with older siblings at the school applying, the rest of the places are taken up by those children. However, not everybody will happen to have a child of the right age.
After the, say, 30 siblings have their places, we are left with 25 places.
The distance between the school and their proven home address is provided (usually by the local authority). They are put on a list with the nearest at the top.
The 25 who live nearest the school get offered places. The other random number who applied, say 40, are put on the waiting list, the closest being top of the waiting list, the furthest away at the bottom.
After this point, some may not take up their places. Those places are then offered to the top of the waiting list in order.
Like I said, it is unlikely that the number of siblings plus the few in the higher standard category and EHCP who are handled separately but come in first will be larger than the size of the year group.