Hello Jan, sorry it's taken a while to reply. Not sure whether you have advanced your search and answered your last question.
Afternoon maddys52 had a quick search in Family Search found Mary Staff's burial records howcome she isn't listed before her brother Samuel Sandall as Sole Beneficial in the will, trying to find when her brother was married, and how many children did Mary Staff have. 
Regards,
Janet 
You can search NSW births, marriage and death records on the NSW gov site:
https://familyhistory.bdm.nsw.gov.au/lifelink/familyhistory/search?14Note that you can search with a wildcard (*) - useful as there are various spellings of SANDELL.
From there you can find Mary SANDLE married John STAFF (3406/1824 V18243406 3B) District "CB" means it was at St John's (Church of England) at Parramatta.
[Early church codes here:
https://www.nsw.gov.au/topics/family-history-search/registry-records/early-church-codes]
There are then 12 births to a John and Mary STAFF between 1825 and 1848.
A Mary STAFF died 1886 registered at Parramatta (8567/1886 ). There is no parents' names recorded on the index, instead it says "aged 82 years" - which fits with your Mary, and implies her parents' names weren't known by the informant and won't be recorded on the death certificate.
Can't see a likely marriage for brother Samuel SANDELL in NSW. There are two deaths for a Samuel SANDALL in 1805 - (728/1805 V1805728 148) and (1998/1805 V18051998 2A).