The only way to tell for sure is thru census, will, cemetary, etc. research. Will of father and/or mother might establish a relationship.
I have the 1851 and 1881 censuses for IOM and indexes for the rest (prepared by our own Brian Lawson).
I'm not Manx but am an avid adoptee My friend has some Lezayre Christians in her lineage.
I'll be away for a few days this week.
Let me know if I can be of any assistance.
Regards,
Donna