I am somewhat skeptical that the James PORTER (d bef June 28, 1824 - Washington, PA) is the son of James PORTER and Elizabeth LYSLES and the brother of Nathaniel PORTER (b 1737, d 21 Sep 1818 - Washington, PA).
James PORTER, of West Nantmeal Township, Chester County, PA, executed a Will on 23 Aug 1774, which Will was proved 2 Mar 1775, naming his various children and no living James PORTER was amongst these. This Will was recorded in Chester County Will Book "F" at Page 77. I have a copy, but had NOT yet transcribed and posted the full text. But I can quickly abstract for you here that James PORTER's will identifies wife, Elizabeth, sons William and Nathaniel, and daughters Rebecca, Isabald, Elizabeth, Elinor, and Violat. No other children or grandchildren are mentioned.
My GUESS is that the James PORTER (d ca May 1824 - Washington, PA) named in the Will I posted whom you identify as being buried in Mingo Cemetery was probably the son of William PORTER (b 1729 - Chester, PA, d 1 May 1803 - Adams, PA) and Sarah PIERCE and the GRANDSON of James PORTER of James PORTER of Chester County.
That William PORTER (b 1729 - Chester, PA, d 1 May 1803 - Adams, PA) also had a son named Nathaniel PORTER (b 19 Feb 1760, d 13 Nov 1813 - Washington, PA) who resided in Washingotn County and predeceased his uncle Nathaniel PORTER by five years. And William PORTER also is known to have had a son named James PORTER, of whom I know little.
I believe that William PORTER (b 1729 - Chester, PA, d 1 May 1803 - Adams, PA) was the ELDEST of James PORTER's children and he WAS mentioned in his father's Will.
I must also point out that the Will of the James PORTER whch I posted also is DATED 9 May 1824 and was PROVED 28 Jun 1824. It would therefore appear that this Will (a) was written in CONTEMPLATION of death and (b) James PORTER must have died between 9 May 1824 and 28 Jun 1824, probably closer to the former than the latter. THis would NOT appear to be consistent with the February 1825 date you assert.
What is the basis for the date you give? Is this actually inscribed on the bural marker for the James PORTER buried at Mingo Cemetery? If so, it would appear that there were two James PORTERs who died in Washington County, PA, very near the same time.
Nathaniel PORTER (b 1737, d 21 Sep 1818 - Washington, PA) also had a son named James PORTER, but this James PORTER clearly died BEFORE 8 Apr 1818 when Nathaiel made his Will, as there is a reference to "my son Jameses heirs".
I do NOT have the full text of the Will of Nathaniel PORTER (b 19 Feb 1760, d 13 Nov 1813 - Washington, PA). Someone else had the microfilm reel out when I last visited the Historical Society of Pennsylvania.
It seems likely that there was also a family connection between Nathaniel PORTER and Matthew PORTER, who was named one of the executors of Nathaniel PORTER's Will. But I have NOT yet ascertained what that family connection might have been. Perhaps Matthew was Nathaniel PORTER's first cousin, or was a nephew -- possibly a son of a son of James PORTER, of Chester County, who predeceased James PORTER and was therefore NOT identified in James PORTER's Will.