I fear the fact that 1st DD is in Year 7 in Townley will weaken our appeal.
I don't see why - it is open to you to argue why your child is better suited to the school you're appealing for.
Besides, an appeal panel won't necessarily know about the sibling. See this item about the CAF and whether the panel is likely to have access to it (and therefore to any information you have given there):http://www.elevenplusexams.co.uk/appeals/general#a21
My first experience at appeal - scared & confused where to start from.
All you need to know is in the Q&As! http://www.elevenplusexams.co.uk/appeals/