Samuel Mortimer was probably a tenant farmer in property owned by the Last's, and as such would be known to Margaret. Samuel had been married at least twice already, having had two children to a Susanna and one child to a Lydia, all in the 1750's. So he was probably about Margaret's age (who was herself born in 1722), and no doubt they formed an attachment during the five years since the death of Margaret’s late husband. Margaret therefore presumably had no qualms in entrusting the care of her youngest children to Samuel, who probably made certain promises to her on her deathbed.
Samuel seems to have immediately gone to live at Weston, presumably taking over Margaret's old home, but it also looks as if Mary Last (widow of Robert at Bruisyard) also sent down to Weston her charge, Anne (mentioned earlier), to be Samuel's housekeeper, (which would get Anne off Mary's hands). Anne would of course take along her own daughter Anne, by then about 14 years of age. On 7th May 1778 Samuel Mortimer married "Anne Last". She already had a six-week old illegitimate child (probably by Samuel). Samuel and Anne had two more children very quickly. Anne died after the last one, and was buried (at Weston, December 1780). It is not clear who this Anne was, but the logical answer would be that she was the one born in 1761 at Bruisyard. If so, then she was just age 19 when she died, having had three children.
After this episode there was probably concern about the upbringing of Sarah (c.1759), Charlotte (1761) and Elizabeth (1766). Their responsibility was probably taken over by one or other of the married siblings, in preference to Samuel Mortimer.