OK, the first thing you need is a copy of the admissions policy and this should be very clearly available on their website someone (legal requirement)
This will list criteria in order of importance, so it might be:
looked after kids
kids with statements
catholic siblings
catholic other kids
non catholic siblings
non catholic other kids
distance.
Or it might be
looked after kids
kids with statements
all siblings
catholic kids
non catholic kids
or some other variation. The point is that if it is the second one, then you should have been above all the categories that come after siblings, which means the class must be full of looked after kids, kids with statements and sibling who live closer.
It is unlikely that there are so many siblings who live closer (but not impossible) in which case they have made an administrative error, and they have to offer you a place. This is the key on which your appeal should be based, and so it is vital to find out the admissions criteria
BUT if it is the first, then above you are all the catholic kids who applied and so you may very well have missed out.
I think I am right in telling you that they have to tell you why you didn't get a place, and that should also make clear if they have used the criteria properly.
Time to get a bit bolshy on the phone and hold on to speak to someone who can give definite information, and stop passing you from pillar to post