Not sure if "Aunt Sarah" (from Upavon) had any siblings. But it does beg the question how a woman from Wiltshire marries two well-off chaps in London and Surrey. How did she meet them?
Perhaps Richard and Jane Maslin, witnesses at Aunt Sarah's 1822 marriage, can provide the link.
Not sure if its related, but a Richard Maslin (born 1786) was buried in St James, Piccadilly on 21 Dec 1832.
I only mention it because Jane Evett (nee Maslin) gives St James as her birthplace in the 1851 census.