This site is a collation of descendants of Joseph Seyler (or, perhaps, Jost Seyler) of Clinton Co., Pennsylvania.
The information comes from various Seyler descendants, both in my line and others.
One family tradition is that James & Ellen Catherine (Allison) Seyler were cousins. While this has never been established through documentation, Ellen's mother was a Seyler so it's certainly possible — perhaps even likely given the proximity of the families and the population levels of central Pennsylvania.
The exact degree, if the relationship exists, is not known. Some family researchers put the common ancestor as a mutual great-grandfather, Joseph Seyler. Ellen's relationship with Joseph is established through her death certificate, her mother's birth certificate, and then her grandfather's death registration. James' relationship is more family hearsay and, in fact, many researchers put his great-grandparents as Johann Jost & Elizabeth (Heckert) Seyler with no indication if this is the same person as Joseph. Unfortunately, the latter is also without documentation that I've seen.
For the moment, I've followed family story and noted Joseph-perhaps-named-Jost as the common ancestor but it's entirely speculative.