Step 1:
An application for 'letters of administration', to dispose of your sister's estate in accordance with the intestacy rules, needs to be made to the Probate Registry:
http://www.hmcourts-s...robate/registries.htm
That application would normally be made by by your brother-in-law but the Probate Registry should recognise his nephew's power of attorney to act on his behalf.
Step 2:
Once your brother-in-law's attorney has the letters of administration he can then distribute your sister's estate in accordance with the intestacy rules. If the value of her estate (including the house) exceeded £125,000 it's possible that not all of her estate belongs to your brother-in-law. It will depend upon the date of her death (whether before or after February 2009), the size of the estate and whether the couple had any children. The rules are here:
http://www.hmcourts-s...te/why_will.htm#chart
Step 3:
The title to the property can then be transferred:
http://www1.landregis.../public_guide_009.pdf
Chris