What is the reasonable limit of the term "relative" on the 1911 census?
I'm trying to prove a complicated theory, and a key part of it is that a man and woman having all their children in the same townland (pop. >200) in the 1820s were brother and sister. From baptisms, the husband of the woman definitely knew the man. The woman at marriage in 1820 is listed as being from the same townland as the man had his very first child in 1809. I infer that he was already living there when he married, but the record only gives the bride's townland.
Two other women with the same surname who married were said to be from the same townland and known to the man. No other men with the surname had children in the townland. Though it is a common name: Brien. Possibly some men may have married out of the townland, but it is not known.
Here's the key part: the 1911 census. A grandson of the early woman is listed as a widower. He has no children in his house, and though he has adult siblings in the same townland (different from the original), none of his children are staying there. I can only trace one. The others might even be in America. A three y.o. girl, daughter of the widower, is living two townlands over, in the household of the grandson of the early man and is listed as a "relative."
Some qualifiers: I know the wives were not 1st cousins, but cannot trace it further, as they came from parishes where the records only go back to 1850 and 1830. Different parishes though perhaps not too distant from each other. No repeat names.
The men on the census were also not 1st cousins, though quite likely were related twice, as they shared a surname, but the closest they could have been along the male line of the surname was 3rd cousins.
Is it reasonable for me to assume (let's say >90%) that the men on the census were 2nd cousins and their grandparents siblings?
I know that Irish people often knew their 3rd cousins, but my theory would make the little children 3rd cousins to each other. And it seems hard for me to believe that anyone with living siblings would leave their child with 3rd cousins, if the heads were only 3rd cousins, and the original man and woman not brother and sister, but 1st cousins.
Thoughts?