I don't have kids so I'm just coming at this with non-parent logic.
They have a safeguarding liability, so worth checking if some guideline somewhere has changed, but they also cannot be ableist. I have to assume that you have a child that cannot be left unattended, and cannot easily be transported from the car. They know you are directly outside the school and you have offered a reasonable solution to a difficult situation.
If what I've written above is roughly the situation, I'd be sending a very brusque letter about the situation, and that if your child isn't released to the sibling, you will leave the premises and the child at that school will need to be dropped to your home.
You are physically at the school. You ARE collecting your child. It sounds completely petty for them not to honour a situation that has worked for years.
I also looked up the law on this and it's pretty clear - as long as they have it clearly documented in writing from you that the sibling will physically collect the child, they cannot legally stop this.
FROM NSPCC WEBSITE:
"There is no law determining the age at which a sibling can accompany a child to school. It is ultimately up to parents and carers to decide whether they feel this is appropriate.
Factors that parents and carers should take into account include:
- the maturity of all the children involved
- the length and nature of the journey home
- the behaviour of all the children involved
- the relationship between the children collecting or being collected.
Some schools prefer not to send a child home with a sibling unless the sibling is aged 14 or older."
So, a school may 'prefer' not to release to a sibling under 14, but they cannot stop it.
https://learning.nspcc.org.uk/research-resources/schools/dropping-off-and-picking-up-before-and-after-school
(Edited to mark the actual quote from NSPCC site clearly.)