Most people, in my direct experience*, are not as rigorous in their 'research' as they think and are happy to accept tenuous links. So, I tend to be cautious of anyone who claims to have 'done their research' and taken their family history back half a millennium or more. Serious historians will often take decades to verify a genealogic line, especially if it is for someone from the 'common class'.
In the UK one of the aim points is how close you can get to the Domesday records written in 1086. Few come anywhere near it.
As for DNA tests, a leading commercial enterprise in ancestral DNA replied to the following question which tends to suggest that DNA genealogy is not the magic bullet.
Can siblings have different ancestral DNA?
So yes, it is definitely possible for two siblings to get pretty different ancestry results from a DNA test.
Even when they share the same parents
emphasis mine
I put all this here in the hope that people will exercise sufficient caution with regard to historical DNA matching and its use in genealogy.
*my direct experience - my brother is a genealogist of some standing in the UK. To the extent that people from all over the country write to him to ask not only for his advice, but for access to his extensive records. We have a tenuous link on one line to a crewman on the Bounty. While this is exciting stuff it cannot be definitively verified and so remains just a fun story.
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