Hello Wendy,
As Donna says Dinah is not a common name.
There only two Dinah Cottiers in the IGI. Yours and Dinah Cottier Kelly.
My guess is that Dinah was wrongly entered or transcribed as Diana.
There are even fewer Dianas, and in fact, there is only one Diana Cottier in the whole IGI. She was the one christened in Marown on October 30 1812, daughter of John Cottier and Anne Crellin. John was probably the brother of the William living at Eairy Jorey in 1851, and the son of James Cottier and Elinor Cretney. It is likely that Dinah Cottier Kelly was the sister of James, and that your Dinah was named after her great-aunt.
I also think that Caesar was probably Dinah's sister but the IGI shows Caesar Cottier Crellin.
There was a James Cottier who was christened in Marown in November 1753, son of John Crellier and Ann Christian. A LDS submission for Silvester Cottier b 1755 shows him to also be the son of John C. and Ann Christian.
Good luck
Bruce