I have checked the LDS site and your family are there. John Cowley married a Maria Catherine Kelly 9/April/1850 Kirk Braddan (Douglas) and had several children. Rebecca was christened in 1854 (not 1858) and there is a son John Edward who could be the witness.
So the family should appear in the 1851 & 1861 IoM census records. If you post a message some kind person may give you the details. This will help give the place and approx. age of John & Maria.
In addition some-one may be able to help with the details of the marriage entry in the parish register.

Good Hunting!