Hello cousin! :)
My great-grandmother (Joseph France's niece - her mother was his sister Anna) was also named Sylvia. I've always wondered if that was a family name or just a popular Czech name for girls.
I still wish I could trace the Frances back more. All I know is that they came over from Bohemia (now Czech Republic) and would have been named Frenc at that point. The women were Frencova (or sometimes Francova), while the male surnames would have been Frenc/Franc. The most I could find was their parents were John and Anna. John doesn't appear to have come over to the US, but Anna came to Chicago with Joseph and 3 daughters, where they settled. Anna Frencova died in 1901. I have her obit from Chicago's Czech paper, the Denni Hlasatel. If I have translated it correctly, it only lists her children in the US, and says she has 22 grandchildren. The 1900 census showed her as having 8 children, 5 still living. Which leaves a 5th child that may have stayed behind in Bohemia with her husband. It also lists "Jan Frenc, her grieving husband," so John was still alive... somewhere.
Feel free to send me a message if you have any other questions about the Frencs/Frances, or wish to discuss them. further. They are still somewhat a mystery to me as well!