The eldest son is often not mentioned in the parent's will because of the Manx law which requires inherited property to go to the eldest son automatically, or eldest daughter if no son, usually after the death of the surviving mother/father. If they die within a year of marriage the property reverts to the original family, unless there is a child for whom provision is made.
Sometimes children are not mentioned when they have had a marriage settlement, and these are usually the older ones.

If several children have been mentioned you can usually identify the correct parents from their bapts, especially after 1841 (censuses).

Sue