That's really, really unlikely. It's not going to affect future legitimate applications with the school, either. If the address was not changed and the same person was down as a contact as well as applicant (the eAdmissions system only allows for one parent to be the applicant, for example), that would link them on the school system.
Unless this is a second home address for a SC, not updating it would be absolutely ridiculous for the parent making the application - if nothing else, because it could be used as evidence by the other parent making a claim for child benefit or that the child actually lives with them permanently/should have an order made that they live with the parent because all evidence shows that to be the case.
Not that I encourage this sort of thing, but rather than the dire warnings of the OP being done for fraud and scaring her, if anything, it would be to her advantage in the future due to sibling criteria usually being either 'living at the same address'. Obviously, don't do that, that would be false details and the younger child could have the place withdrawn, but having her address used without her consent is not in any way going to get the OP in trouble now. Assuming it is the step family situation, at any rate.
Most LA systems automatically cross reference applications to council tax accounts where the number is provided these days, as it's faster and more cost effective than paying staff to sit there sifting through badly scanned documents. The sooner they all catch up with this, the better.
By all means, the OP can notify the council and say 'I've received a letter but the child doesn't live here' and keep her head down when the shit hits the fan. But she's not at risk of getting in any trouble herself, now or in the future.