Hello tiredmum,
I am not at all familiar with your particular area but will try to answer some of your points on a general basis.
When you appeal for an oversubscribed school you may use any arguments which demonstrate why that particular school is the most suitable for your child. You will find plenty of examples in the Q&A.
Talents and interests which fit well with the curriculum, extra-curricular activities, specialism and ethos of the school are relevant and it is useful to include some documentary evidence though perhaps not to the extent sometimes used when people submit every certificate their child has earned from toddlerhood onwards...
Try and concentrate on what your child will get at this school which they will not be able to get at the allocated school. Are there subjects they can study which would not be available otherwise; does the school have facilities not available elsewhere, offer opportunities for particular activities etc, etc? Is the journey easier? Will there be a friendship group there that will not be at the allocated school?
Whilst letters of commendation from primary school or others are very nice, they really need to say something which is relevant to the child in relation to the school being appealed for to carry any weight.
Sould i mention her sister at another grammar?
I can't see that in itself this would be significant. You might, however, give a mention to the fact that your child was determined that she wanted THIS school and why even though she was familiar with another.
Should i mention kent blunder?
I don't think a mistake in a different exam would add anything to your arguments.
Also i read in a thread that rank order admissions schools need academic evidance on over subscription appeals ? my head mentioned a while back that this was NOT the case, so bit confused
I think this is rather a difficult one. Whilst they are oversubscription appeals I think there can be a place for
specific academic arguments and/or for mitigation arguments where you feel the child would have scored higher but for particular circumstances. For example, it might be that the child has not scored particularly highly overall because their talents are rather one-sided and they are a genius at maths but not so good at English; and a distressing personal circumstance may have affected a child and depressed their score.
However, it is not necessary to present general arguments about academic ability when the child has already proved it by passing the exams.