Nailpolish - Sorry this is long but thought it might be useful:
my ds has just started Reception at a Catholic school and we are not religious at all. He has never been Christened.
The school admission policy is stict but in our area they can't fill the intake (30) with Catholic children so despite being about 8th on the criteria list (after Catholics, siblings, other Christians, other faiths etc...) we still got him in. In fact I'm not sure but I think they have a policy of taking 15% non-Catholics. I had to write to the Governers explaining why I wanted him to go to the school but I didn't pretend he was Catholic or anything like that. (They would have wanted a Baptism Cert anyway).
We sent him because not only is it the best educationally in my area but it's whole ethos is about understanding of, and tolerance of, others and I can wholeheartedly support that. They also emphasise nurturing the individual and teaching respect which is what I wanted for him.
They teach National Curriculum RE obviously so they do get to hear of other faiths but I must admit I hadn't realised until he started that they do Mass, have the local priest in and do all the prep for those taking Communion. Maybe I was naive but so far so good. I'm still glad we got him in there.
I don't know if it's a local thing or a national one but in our LEA you have to state your 1st, 2nd, 3rd choice school. The schools all see these choices and will fill their intake from the 1st choices first - i.e. if you fail to get into your 1st choice scholl, then you risk not getting in to the 2nd choice either because it may have filled it's intake with children who put it down as 1st choice. Does that make sense?
Your sister's LEA website should have details on whether each school usually fills it's intake or not and what the school's selection criteria is.