I'd ask the schools for their stats for the last 3 years or so. They should be able to send you admission stats which will tell you:
How many places went to siblings. In our area, at all schools we looked at, about half of all places in any given year go to siblings. So then you know that there are 15 places left (say, if the intake is 30 pupils).
The stats will then tell you how many people got in under the other rules (eg being a Catholic, then non-Catholics), and what the maximum distance was under under each rule (if distance is used a tie-breaker if more people apply under a certain rule than there are places).
Then you'd know the likelihood of your getting a place under which ever rule you'd be applying under (our council has a "distance finder" which measures to the metre how far you live from each school).
You might find that all places went to siblings and Catholics who lived under 400m from the school, and you live 600m from the school, in which case you are probably not in with a chance.
Or you might find that all places went to siblings, Catholics 700m from the school, and some non-Catholics, in which case you are probably in with a chance.