I have also wondered about this purported lineage, supposedly through Jackie's father's line. I have done some research on this and so far have found nothing to support a link between Jackie and Anthony Jansen van Sallee. Jackie did have a cousin though (Mary Lee Ryan...a daughter of her mother's sister, Marion Norton Lee-Ryan) who married William Amherst Vanderbilt Cecil and HE was a descendant of Anthony van Sallee. Their children (Caroline Kennedy's 2nd cousins) would also have descended from Anthony. It is possible that someone incorrectly presumed that if these relatives were descendants of Anthony that Jackie must have been also.
Also of interest is that Aristotle Onassis' first wife, Athena Livanos, after her divorce from Aristotle, married an Englishman...John George Spencer-Churchill The 11th Duke of Marlborough and HE is a descendant of Anthony since his grandfather, Charles Richard John Spencer-Churchill - The 9th Duke of Marlborough was married to Consuelo Vanderbilt (the 11th Duke's grandmother )and SHE descends from Anthony. I am still searching though for a tie to Jackie!