Well, that was rather disappointing. Viewed the will of John Mailer of Cornhill (the one dated 26th Feb 1845) on SP and it mentions no-one other than a brother, Peter, who died two years previous. Was that really worth spending 10 credits?
However, some of the pieces are coming together and this is my thinking now...
CornhillJohn Mailer (1775-1840) married Margaret Smitton (1770-). They had ten of a family:
Helen (1810-1874) - this is the Helen that married William Caw
John (1812-)
Catherine (1813-1826)
Peter (1817-1855)
Ann (1820-)
Elizabeth (1822-)
Mary (1825-)
Catherine (1827-)
Janet (1831-)
James (1833-)
Peter married Mary Ann Cairns and continued to live at Cornhill until his death in 1855. There is a will on SP for Peter Mailer of Cornhill dated 1856. Mary continued to live there for a while as shown on subsequent censuses and there is also a will for Mary Ann on SP from 1908 which shows "Mary Ann Mailer or Cairns, Cornhill, Auchterarder, afterwards...".
I think ownership of Cornhill then passes down to their eldest son, John - I have a photo of a headstone detailing "John Mailer of Cornhill, died 7th Aug 1924".
Helen's death certificate shows John Mailer and Margaret Smitton as parents, so Helen and Peter must have been siblings at Cornhill.
I'm not sure why eldest son John didn't take over at Cornhill, perhaps he wasn't interested, died young, went to farm somewhere else, wasn't able to work, or maybe he worked alongside Peter.
Wester EindJohn Mailer (1774-) married Janet Couper (1770-) and had six of a family:
Mary (1797-)
Jean (1800-)
George (1801-1862)
John (1804-)
Janet (1806-1844) - this is the Janet that married William Caw
James (1809-)
There's an OPR linking Janet to John Mailer and Janet Couper. George's death certificate also lists the same parents so obviously they must be siblings.
Scotland's People list a will for George Mailer at Wester Eind dated 1862 and one for John Mailer dated 1876 so it would appear Wester Eind either passed over to a younger sibling or probably more likely to George and wife Janet Sheddan's son, John.
Unfortunately there are no wills that I can find on SP that would be for the previous generation, i.e. for either of the John Mailer's born in the 1770s which might have mentioned a beneficiary, or given a hint as to how the two eldest John Mailers are related (if at all).
I don't think any of the four other wills I've not yet viewed are going to reveal anything significant as they are really of the next generation down.
So apologies for being stubborn and not accepting Helen was from Cornhill and Janet was from Wester Eind as the information I had seemed to point to the reverse of this. From investigations it would seem this was the wrong way round from the start.
How does this look now?