When does sibling priority begin?

[HopefulNavigator]

In this scenario...DS1 currently Year 4 and on waiting list for primary school X. DS2 will very likely get a Reception place for Sept 2018 due to distance from school X. So at what point will DS1 move up the list according to sibling priority..is it April 2018 when DS2 is officially offered a place or Sept 2018 when he starts Reception.

Talk to the Admissions authority. The admissions code doesn't stipulate, other than the list needs to be run along the same priority as admissions. Different authorities take a different approach to this.

You also need to check the criteria for the school carefully. Some only give sibling priority when the sibling is younger than the child already at the school.

In my LA it is once the sibling starts school that the other siblings get priority - so September 18 in your case.

At our school it is worded as 'children who have a sibling at the school at the time of admission'. To prevent current siblings but who will have left by the time the new child starts. So arguably it doesn't matter if you have a sibling at the school right now, it matters only if you will have a sibling at the school 'then'. (It would seem inconsistent to not give priority to children who have a sibling at the school at the moment, but won't at the time of admission, but neither give priority to children who do not have a a sibling at the school at the moment, but WILL at the time of admission. Both depends on the future panning out regularly. In both cases it may or may not.)

So according to this logic, your DC2 should IMO get sibling priority for any place that comes up for September 2018, from the time your DC2 has accepted a September 2018 place from the school. So if a place becomes available in June 2018 but is filled right away, your DC1 won't get sibling priority yet. But if it is only offered for start in the new school year, you could IMO expect the sibling priority to apply.

However, these things don't tend to always be logical or make sense; best to ask the LEA or the school's admissions body.

It's once they start and are officially on roll.