What I have is as follows:
Wm Caine of Glan na gey married Mary Cannell in 1713. They had 3 children John b 1715, Mary 1717 and Thomas 1723. In 1724 Wm Caine died. His Will mentions his wife and children as above.
His son and heir John married Christian Cowley in 1729.
So I'm assuming that the John Cain who married Alice Cannell [if he was b 1722] can't be the son of John and Christian.
There doesn't appear to be a baptism for John son of John and Christian and he is not mentioned in his gmother's [Cath Cannell Cowley] Will 1740 but he is mentioned in his gfather Wm Cowley's Will 1744.
If he was only born C1740-44 could it be him who married the no 2 Alice Cannell in 1868 and therefore I have the connection for Isabel Cain??
I'm glad I typed that - and quite happy for anyone to pick holes in the theory.