petenas, it doesn't sound like OP is more affluent or wealthy if she doesn't think that she'll be able to live in this catchment very long.
She just sounds like someone trying to do the best for her children, within an inherently flawed system.
You do know that people move for lots of reasons, don't you? The impending changes in Housing Benefit are going to force 1000s of families to relocate from where they've build their lives and community, people move for jobs, childcare reasons, because they get evicted, because they need more space etc etc.
It's incredibly reactionary to assert that one particular family has more 'right' to a particular school place because they happen to have lived in a particular area for longer.
The Census information only provides the barest of school planning functions in areas of high mobility like many urban areas. Talking of which, successive governments have known for years the birth rate rose significantly in the late 2000s, creating a demand for 100,000s of extra reception places, although this current job are too busy with their pet projects of free schools and academies to be planning in any sort of strategic way for this.
The rules specific a 'permanent address'. Do you think it's more moral to stay at a particular address where you may have lived for some time until you've got your first child into your preferred school, then move knowing that you can take advantage of the sibling rule?
Genuine questions by the way.