As you saw in the two will transcripts you requested (William Kermeen d.1751 & Ann Kermeen als Christian d.1753) William also referred to his son John Kermeen, as do other family wills. I don't think the agreement was strange - there are many wills which have similar clauses when someone for whatever reason hasn't claimed their inheritance.

If John had been a half-brother he would have been referred to as such - usually brother "of the half blood", especially when named as next of kin in a decree. He was referred to several times as just one of the uncles and aunts in the Maughold 1743 decree of his sister Ann Stole als Kirmeen. On the other hand, the Maughold 1760 decree of William Corkill "hath Decreed Margret & Cath: Corkill, sisters of the whole Blood sole & joint Adminrs of all the Goods & Chattells that descended unto him by the death of their mother Elizth Corkill -- and John & Thos Corkill brothers of the half Blood, with Margt. & Cath: Corkill aforesd joint Adminrs..." etc.

To make judgements I think you first need to look at a family as a whole, with the information from all family wills, land records, and parish registers for a particular generation. In this case I think the term natural son was used that once because it was in a sentence about shared inheritance between son and son in law.

Sue