The name Isaac seems to feature strongly in the Williams family. Both my great-grandfather and 2x great-grandfather were called Isaac and my great-uncle was Isaac Leonard. The name was also given to children of several of the daughters who married into other families in the 19th century.
It looks as if the name Isaac actually came from the SUMMERHILL line via Charlotte Summerhill who married James Williams sometime before 1836. The earliest Isaac I have is Isaac Summerhill, who was born and died in 1780 in Frampton on Severn, Gloucestershire. He was the son of Stephen and Sarah Summerhill. 7 years after Isaac’s death, the parents named another of their children Isaac, a common practice to ensure a favoured name survived in the family.