The Lewis family has a tradition that their ancestor Mary (Goldsmid) Lewis, wife of William Lewis, was a mistress of George IV, King of the United Kingdom of Great Britain and Ireland and that George was the father of some or all of her children. Mary is said to have married William Lewis in 1810 after her affair with George IV ended, but it might be significant that she is said to have already been "Mrs. Mary Lewis" during the affair with George IV.
It's not always easy to prove relationships that depend on family tradition, but interesting to look for background. Was Mrs. Mary Lewis really a mistress of George IV? Is there any contemporary evidence that he was the father of her children?
Even if we can't find primary sources at this point, it should be possible to discover when the claim was first made and who made it. Then, we can begin to evaluate whether the claim is plausible.
Here's a blog entry that reviews the story:
http://miriamhakedosha.blogspot.com/2008/11/simeon-levi-west-anglic...
Something here might provide a clue to other researchers.