AFAIK, Lower Sorbs are pretty much all germanized by now. Only Upper Sorbs still survive in larger numbers.
You know, most of the Irish can no longer speak Irish, but nobody sane will use their DNA as English references.
And you are suggesting to use a genetically pure Lower Sorb who just lost her language, as a German reference.
As Leto noticed now this "Brandenburg_outlier" will show up in models for a lot of Eastern Europeans as their supposed "German" ancestry. For example I can be modelled as "German_Brandenburg_outlier" plus Dutch, because I'm more western-shifted than this sample.
BTW you know very well that Beeskow is in the region of
Niederlausitz, but you called it "German_Brandenburg_outlier".
You could at least call it "German_Niederlausitz" (and probably not such an outlier for local Germans from Niederlausitz).
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BTW, all of academic Sorbian samples are Upper Sorbs from one town, Kamenz:
https://en.wikipedia.org/wiki/Kamenz