Hello and welcome!
I have a daughter in Y8 at Alty Girls and another one going into Y7 in September. When No 1 sat the exam (in 2006 for 2007 entry) the catchment area consisted of 3 concentric circles centred on the school and, basically, girls in the circle nearest school got in before girls in the next circle etc.
When No 2 took the exam (in 2008 for 2009 entry) the catchment consisted of one very large circle (taking in Knutsford, parts of Warrington, parts of Salford, Didsbury, Wilmslow and parts of Stockport) and again girls nearest school got in before girls further away.
It has now changed to the 8 mile radius which, I suspect, is not that different from last year's circle, just a bit more accurate since it is a definitve distance rather than someone arguing her house is this or that side of the line on a tiny map.
My interpretation of the change from No 1 daughter to No 2 daughter ( and to this year's 8 mile version) is that for a few years there will be a knock on effect on sisters being admitted and then the impact will be reduced. eg in No 1 daughter's class there is a girl from W Stockport who would have been in the outer concentric circle when she was admitted. Her sister will be starting in Y7 in September this year having got in on the new sibling rule before any of the girls living in the Altrincham area who don't have a sister already at the school. However, based on the fact that the elder Stockport sister got in in the first place from the outer circle I'm not sure that has as much of an impact as it does at first sight.
For what it's worth, my daughter's Y8 class is made up of 1/3 Altrincham girls, 1/3 Sale and Lymm girls and 1/3 girls from Wilmslow, Knutsford, Salford, Manchester, Didsbury, Stockport. That is representative of her year as a whole.