As Nancy was born in 1849, this couldn't be the Nancy I have listed, but there is a very good chance my Nancy is related farther back.
My Nancy was born about 1815. She married Newton Law and their children are as follows: William Marsha Nancy Rosanna Samuel Wilson
Of the relatives you mention, three of the names appear above: Samuel, Nancy and Silson. To me, there are just too many in common (Wilson????) to be coincidence.
I'm leaning toward my Nancy being a sister to your Samuel. My Nancy's daughter, Nancy Law, was born about 1841; her son Samuel was born May 3, 1846.
My Nancy Usselton would be my 3rd g-grandmother. Unfortunately, I have no further information on her. Yours is the first good lead I've had. Do you by any chance know where in New Jersey she was born or died? Or what religion she might have been? I can do some further checking.
Anyway, keep in touch. I really think we have something here. Even more so as you note your gggrandfather, William, was born in Beaver County. That's not very far at all from Lawrence County, where Nancy and Newton lived and raised their family.