As others have indicated, anyone looking after a child under 8 years old (for more than 2 hours per day) is required by law to register as a childminder. (There is an exemption when the child remains in their own home, which is classed as 'babysitting' rather than 'childminding'. There's another exemption for 'occasional' activities, such as when a child has their friends visit for a sleepover, but neither of these exemptions apply to you).
There are probably thousands of 'family arrangements' (such as the one you're planning to get involved with) where the rules are ignored and 'social services' wouldn't want to get involved unless they were aware of a specific problem about the arrangement. Because it's 'within the family', it's probably extremely unlikely that 'officialdom' would either know about your arrangement or be concerned about it. However, you should know that, unless you register as a childminder, you'll be breaking the law.
If you do decide to register as a childminder, you need to be aware that you'll have to pay a registration fee (including the costs of the criminal records check) and show that you've complied withthe training requirement (e.g. in relation to first aid skills). Your house will also be inspected to ensure that it meets the safety requirements.
Having got that out of the way, I suppose that you'd like an answer to your question!
http://www.ncma.org.uk/MainWebSite/ContentPage 9df2c089.aspx?Map=A0612B45B52E53CB05FF6A5FDBB1 6FB5
Chris