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The panel are most likely to ask about siblings if it's relevant to the case being presented.
For example, if parents say they have serious logistical problems getting their children to school, they can expect to be asked how many children there are, how old they are, and which schools they go to.
On the basis of what you've told us, I can't immediately see why you should be asked about your other children, but I suppose there's always an outside chance they might enquire.
I don't think it would necessarily undermine your whole case, provided you have an explanation (which you do).
If you had relied solely on anecdotal evidence, then it might not impress the panel - they would probably be thinking that you really should have satisfied yourself with regard to the accuracy of the information.
However, being misled by the primary school should carry more weight, and I think that is the bit you need to emphasise, even though (I assume) you cannot prove it.