I don't know how I missed your question about more information about the family of Martha (Alexander) Porterfield.
So far as I can tell, based on the will of Rebecca (Alexander) Emmit/Emmitt which was written on 21 Oct 1795 and proved 22 Aug 1792 in Chester County, Pennsylvania, MarthaPorterfield, wife of William, was her sister. At least she mentions her nephew William Porterfield and his sister, Martha Porterfield, who were the children of her nephew Josiah Porterfield. She separately mentions neices Rachel and Mary, daughters of William Porterfield.
Rebecca Emmitt was rather hit or miss about who she mentioned in her will - not all of her relatives received bequeaths.
Rebecca (Alexander) Emmitt was the daughter of John Alexander (who died in Chester County, Pennsylvania about 1750 or so) and his wife, Susannah, who died several years later. Rebecca seems to have been the only one of their children that they mentioned in their wills - possibily because she was the only one who stayed in Chester County.
The Alexander Forum has a number of entries about this family. Let me know if I can be of any help. Sorry it took me so long to answer your question.