Iím not sure from your message whether youíre referring to state or private schools or whether your sister is looking for admissions this coming academic year?
I canít help much with the private schools but have experience of the state sector.
Unfortunately it is not as simple as registering with the preferred options. Applications for reception are made in December/January before the September theyíre due to start. You state six preferences and usually places are allocated according to distance (although there are other criteria that may prioritise such as siblings or SEN). Coincidentally admissions for this year closes today, although itís possible to make a late application.
If the child is older, I believe you make applications directly to the school and the process is slightly different. This is still done on distance so in either case it will depend where your sister lives as to what would be sensible choices.
You can find out more about Southwark admissions here: [www.southwark.gov.uk
If itís private schools youíre interested in, Iím sure someone will be along soon to help you soon!