My eldest dd has been offered a place in Juniors ( year 3) starting in sept 2018.
My youngest dd hasn’t been offered a place for reception in September in the same infant school.
So I want to appeal for a reception place. Would I have better chances to win the appeal if I appeal in sept when my youngest will have a sibling started in the junior school? Admissions has advised me to appeal now, but I don’t think I will have much chance of winning as my youngest doesn’t have a sibling at the junior school now.
your eldest DD is not on role until september at which point you will presumably have a valid sibling link and will go to the top of the waiting list. It won't affect any appeal though as when you applied you did not have a sibling link(you just don';t live close enough to have been given a place). Presuming no error was made in your application then you are unlikely to win an appeal - you best chance would be to get in from the waiting list.
The presence or otherwise of a sibling won't make any difference to your appeal. If this is an infant class size appeal you can only win if they have made a mistake. At the time the decision was made your youngest did not have a sibling at the school, so no mistake was made on that count. It doesn't become a mistake just because the older child is admitted after the decision was made.
If this is not an infant class size appeal you would be trying to win on the balance of prejudice. Whether or not your older child is at the school would be irrelevant.
It won't make any difference to a potential appeal as the school is full so your grounds on which you can appeal will still be the same. Having a sibling doesn't give you automatically the right to a place if the school is already full - it will just move you up the waiting list.
Make sure your child is on the waiting list, and then double check in Sept that they move up it as soon as your eldest starts (assuming there is a sibling link in the criteria). Whoever runs the waiting list (council or school depending on what type of school it is, either way it should be held until at least December) should do this automatically but it is always worth checking.