I found evidence that this Rachel Lyon, born in 1866, is the one who married David Cornell and that her parents were Isaac Lyon and Angeline Fullerton. She later used the year 1864 as her year of birth which led to some doubts that she was David's wife, but she was present at the death of her father, Isaac Lyon, in 1887. One of her sons also died there 5 days later. Her tombstone and her obituary indicate she was born in 1864 but this is probably not correct. The census of 1870 indicates that she was 4 years at that time and her parents were not married until 1865 since her father was away in the Civil War. I don't know if she did not recall the year of birth or falsified it. At the time of the 1870 census it is difficult to imagine that they would have indicated she was 4 if she was really 6.
Lucy was too old to have been the child of Isaac and Angeline. She may have been a cousin or other relative.