"Not to forget is that siblings don't necessarily show the same percentages of different ethnicities, unless they are identical twins, etc."
This part just does not make sense to me! My sister and I are whole siblings. We both descend from the same people. If we both took the DNA admixture test, and it showed different results for us both, then to me, there are problems with the validity of these tests. Also, another thing I can't understand is why certain people, who are clearly of African descent as well, are showing up as having no African ancestry. If a test told me that, I'll probably say, "all of this is just a bunch of hogwash", put it aside, and gone on with my life as an African-American. That's just me though.