................ You are correct, Squire Thomas did have a nephew (son of his older brother John) named Thomas born in 1755-1843 which married Nancy Nelson. Thomas was one of the first Revolutionary War volunteers from Forks Township having enlisted on August 1, 1776 and having served as a Sergeant under Capt. John Nelson. He also reenlisted on Jan.8,1777 and served for two months as an Ensign under Capt. Nelson. My great/great/great/great/great grandfather, Squire Thomas (1734) was too old to be a soldier, so he more that did his part as a member of the Committee of Correspondence and in other ways. ................
At least this shows the name Nancy was introduced into the family. And the dates seem to match well, which means That your Nancy **could** be the daughter of this Thomas and Nancy Nelson.
The PA silliman line was established by Thomas Silliman who came from Ireland. One of his sons was known as Squire Thomas silliman, my GGGGGrF. I use him as a reference point. My Grandmother, told me that Squire Thomas piloted George Washington across the Delaware River.
The Silliman/Sillyman/Sylimon, etc. line is difficult to trace because they kept using the names Thomas, John and a few others. They still seem to until this day. See the link I am going post as separate thread which shows a picture of a present-day Tom Silliman at work.