Place offered from waiting list

[Natwild]

My child got his first choice school our catchment school from waiting list so am told soon as he starts reception his sister will be first on the list too. She's currently 3rd the other 2 kids have been waiting 2 years for a place I moved 3 months ago but he doesn't start until 17th September but asked if he can start sooner he will miss his first weeks with his new classmates where still in August plus I need his sister in the same school if a place came up the weeks before she will miss out there 2 miles apart 😢

Am not sure what you're asking here?

Why doesn’t he start till 17th sept?

Ask what his official start date is.
17th is likely to be a phased actual start date for reception.
But you have to accept you may have two kids in different schools for a while even when she is the number 1 on list

He may not start in class till 17th if settling in slowly but from 3rd September or whatever he is a child on the school Roll and presumably being funded from day the school opens.

Pupil admissions said to inform them when he has started school then his sister will be first which is a lot better chance both schools are excellent just start times clash