When thousands are comparing their DNA from the same group, you can generally say what markers are evident to a certain degree.
It depends what you mean by markers
Yes as I said the Y haplo test can give you clues to near or distant cousins and you can compare results and see that you are probably related, but that is only a test for men and only a test for one direct line (father to son), so it cannot tell you about the origins of that mans ancestors on all the other lines he has
But this is not how the autosomal test, aka the ethnicity test, works at all
No it's not always reliable as the Spanish marker I mentioned, I don't have any but other family members do.
What do you mean by Spanish marker? Is this the ethnicity test (autosomal)
Those from or associated (married into the old families) are usually pretty similar.
I have a cousin with 25% Jewish, not sure which. I don't have any, but we share lots of other well known markers and we follow the migration pattern.
I think you are talking about the ethnicity test (autosomal) . These tests are not compared to other people who have taken the test. They are compared to a stock sample. This stock sample is tiny and in some cases only a dozen or so samples they already have. They are not other testers samples
25% is bordering on low, so could be inconclusive and not mean anything
The autosomal/ethnic test, the ones that give a geographical area by percent, are unreliable