Inheritance law and autistic grandchild

I wonder if anyone knows a specialist legal advisor who can help with a very specific issue or where to find one ?

My sister had a fairly normal situation. Became full time carer of autistic child.. Her parent contributed some badly needed money all the time. Left money within will when died. Stepparent wants her money. Many complexities result which could fill a page. . One in particular is uncommon. Apparently poverty can be claimed by carer but where caring for a child would make life more expensive (holidays, home security, eating T shirts daily, suchlike challenges)  it is said this this extra cannot be taken into account.  This is apparently the law under section 2 of the 1975 act or at least that is alleged. 

I suspect there are a very people who have the detailed knowledge to immediately recognise this issue and know what to argue against this. Find them is proving hard. I wonder if anyone has any ideas based on experience with this issue?

Also as a warning, if you are helping out a child with carer challenges, do make sure your will is fortified beyond all normal lengths you might want to go so.