Hi Janet -
Sadly, I have no additional information for you at this time, but I'm also a descendant of Joseph SIDDALL and Elizabeth BROOKS and just recently came to the conclusion myself that he's likely related to Francis SIDDALL and Sarah TERRISS/TRERICE (if not descended from them). I'm interested to know how you determined he and James are linked with Francis and Sarah - I don't have nearly as much information as you so my conclusion is based mostly on naming and migration patterns and my gut... :)
One more interesting piece of info to support the family connection is the 1851 census for Joseph and Elizabeth SIDDALL in Mosa Township. It lists a 25-year-old laborer named Daniel TERRISS immediately after Joseph and Elizabeth's daughter Rhoda, suggesting he was a member of their household. Other information suggests Sarah had a brother Isaac with a son named Daniel around the right age, meaning the Daniel TERRISS in Joseph SIDDALL's household in 1851 could be his cousin.
On the other hand, the marriage bond for Francis and Sarah was taken out in October 1813. With Joseph allegedly born in March 1813 (per cemetery records) or earlier, is it possible Francis had a prior wife? Or is there any chance in that place at that time that Francis and Sarah already had a child when they married? Or could Joseph and possibly James have been nephews rather than children?
I'm excited to hear if you're ever able to make a solid connection with Joseph and Francis/Sarah!
M.