The Childcare Act 2006 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.
So what your Step MIL as a relation to your children? I think that your Step MIL is your DP's Mother (is that right), which thus makes them your children's Step Grandparent I think - does anyone understand all these names?
So if they are a Grandparent to your children, then they aren't full blood, but are they half blood? Are you married to your DP, or only co-habiting?
Please can read page 19 of HMRC Publication: 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.
HMRC and Ofsted are both aware that relatives are trying to register as childminders only to care for their grandchildren, nephews etc. How they actually check-up on that, I don't know... but I suspect that if when doing registration inspection or subsequent inspections the Ofsted inspector gets suspicious that the children in the persons care are ONLY relatives, that then action maybe taken.
It may be risky, but exactly how risky I don't know. Certainly HMRC, DWP and Ofsted are aware it occurs... so I would suspect that there are things being done to spot these occasions when a childminder only cares for relatives.