Tom, their children from John b.1799 onwards appear to have been baptised at Saint George, Douglas (John on 10 Nov.1799). The film of the parish register of St. George's Church (Douglas), 1743-1895 is no.0106717, and can be ordered to view for a small fee at a LDS Family History Centre (under Church of Jesus Christ of Latter Day Saints in phone books). This film covers Baptisms 1781-1853 Marriages, 1786-1896 Burials, 1790-1892.
If you look through these registers for all the children's baptisms, you might find additional details of place and occupation which could identify the correct burial dates for John and Isabella Brew, and if found you can then see whether they left wills. You might also find marriages for the siblings who didn't leave the parish.

Ellinor, the eldest daughter of John Brew and Ellinor Corlett appears to have been baptised pre-marriage on 6 Feb.1833 at Saint Mathews, Douglas, so you will need to order that film as well to look for additional details. It is LDS film no.0106718, which contains Baptisms 1705-1848 and Marriages, 1730-1753. I couldn't find John William b.c.1834.
Perhaps he was born in England, and they married there?

Sue