If you can locate Jane's will, it will almost certainly mention spouse and offspring.
However, I did not see her will on Brian's wills index.
Ther is a burial record for her dated Feb 23 1821 in Braddan aged 38. That would give a birth year of c. 1783 and make her about 24 when she married in 1807.
It is likely that James was of a similar age and born between 1777 and 1787.
There is no obvious death record for a James Cain between 1815 and 1851. It is possible that James remarried.
Note that the IGI has an Ancestral file showing James married to Jane and father of Matthias) being born in 1786 at Boilley Vane Farm, East Baldwin Bradden in 1786, the son of John Cain and Isabella Key. Someone did some reseach to put this together and it should be worth checking out, if you have not already done so.

I could not locate a birth record for Jane.
There is a 1851 census record for a Matthias Cain, Builder unmarried, 36 living at 5 Dolton Terrace in Onchan with his with his sister Elizabeth Cain, unmarried aged 40. No place of birth is given fo either and I was unable to identify their parents.

Bruce