Hi Gella,
In total we were there for 10 years as our daughters are 3 years apart. When they say they don't gurantee sibling entry, they really mean it and it is not arbitrary, but for good reasons. The vast majority of siblings do get in by really only if the school feels that they would thrive. I have seen at least three instances of younger sisters not being accepted, and two instances of siblings who were accepted but ended up leaving the school because it did not suit them. I would say in the latter instances, the school seems to have done everything they could do to assist and provide additional support and it would have been up to the parents to decide to move (because their DD was unhappy). And girls have been held back a year as well, rather than leave. Obviously it is disappointing for parents who had their hearts set on having all their girls at one school but at the end of the day I think the school does what they feel is right, and it is impossible to get it right 100% of the time especially when assessing girls at the age of 3. There is always the option to reassess for entry in year 3 of course but I don't know of any cases where girls who did not get in in reception then joined in year 2, so presumably they were happy where they were.
I just am a big believer in the staff and they do have their hearts in the right place. The GDST is also a wonderful larger network that they can tap into for resources and best practice. (I swear I am not being paid, I just drank the kool-aid - happy customer).
Over the years what has happened to KPS and all London schools is the number of applications has gone through the roof, fees have gone up and the demographics of the school has changed a bit as a result. Whereas for DD1 everyone lived in SW6 or SW15, over the years more and more from SW10, SW3, W11, SW1 and sometimes further afield (although to be fair those are usually transferring in for occasional places in years 4 and 5). I do believe they still try to prioritise those living locally, but this may have changed (whether officially or unofficially is anyone's guess). Still, I would say most girls do live fairly locally although lower down the school I could not say.
We were lucky that both girls did well there...it happened to be closest to where we were living at the time as well so maybe that helped.
Sorry for long, rambling post.