Hang on... no one seems to be mentioning issues regarding it being care of a relative.
As I read it michymama's sister is asking her to care for michymama's nephew or niece.
Please can read page 19 of WTC2 section titled: Childcare Provided By A Relative.
Childcare Element of Working Tax Credit cannot be used for childcare provided by a relative, unless that relative is a Registered Childminder who is ALSO looking after other children.
So to answer michymama's question:
Can u become a registered child minder just for one child ?
The answer is NO, if that one child is a relative.
Because in the Childcare Act 2006 it says,
18 (4) ?Childcare? does not include care provided for a child by? (c) a relative of the child;
The Childcare Act 2006 also defines what is considered to be a relative,
18 (8) (c) ?relative?, in relation to a child, means a grandparent, aunt, uncle,
brother or sister, whether of the full blood or half blood or by marriage or civil partnership.
Link to Childcare Act 2006 (PDF)
Therefore (In England) to care for a relative, you don't need to register as a childminder and you can't be paid/part-paid via Childcare element of Working Tax Credit.