The marriage record for Jane and Thomas Muncaster should tell you if she is a widow or not.
Assuming Thomas Cain died, there are two possible burials (
www.lawsons.ca) that could be your Thomas Cain (between 1834 when John was conceived and Aug 1837 when Jane married Muncaster):
CAIN, Thomas 39 Jur 28 Oct 1835
CAIN, Thomas 37 Mal 30 Dec 1836
The second seems more likely since it happened in Malew which is where Thomas and Jane married and where their children were baptised.
Interms of Wills these are the likely entries:
CAIN, Thomas 1836 Dgs A 9 0106248
CAIN, Thomas 1837 Mal A 17 0106248
CAIN, Thomas 1837 Mal E 1 0106437
With it probably being one of the last two. (NB: the 7 digit numbers at the end are LDS film numbers and the number preceeding it is the reference number within the film.)
While on the IGI I also noticed these baptisms for father = Thomas Cain, mother = Jane Kennaugh
THOMAS CAIN - Christening: 30 NOV 1823 Malew, Isle Of Man [two infant Thomas Cain's listed in the burial index for Malew in 1824]
ELIZABETH CAIN - Christening: 25 DEC 1825 Malew, Isle Of Man
THOMAS CAINE - Christening: 22 JUN 1827 Malew, Isle Of Man [perhaps a mistranscription or double transcription of the 1828 entry?]
THOMAS CAIN - Christening: 22 JUN 1828 Malew, Isle Of Man
FRANCES CAIN - Christening: 23 NOV 1830 Malew, Isle Of Man
JANE ANNE CAIN - Christening: 15 APR 1833 Malew, Isle Of Man
JOHN CAIN - Christening: 26 FEB 1835 Malew, Isle Of Man
JOHN CAIN - Christening: 03 JAN 1836 Malew, Isle Of Man
(There were another two burials for adult Thomas Cain's in the index but for much older men are unlikely:
CAIN, Thomas 69 Onc 14 Apr 1836
CAIN, Thomas 68 Mal 7 Mar 1837)
Hope some of this helps.