My Serbian girlfriend told me today about the Sorbian people, who live in eastern Germany, saying they were an offshoot of Serbs, who ended up living to the north. I can't find anything to corroborate this. They seem to be more closely related to Poles and Czechs. Their language is western Slavic, while Serbian is southern Slavic.
While I am sure all Slavic peoples are related at some point if you go back far enough, was there a point in history where Serbs branched off from Sorbs or Sorbs from Serbs?
Fredegar's Chronicle from 7.-8. century mentions Sorbs pretty much where they live now, which was on the border of Francia back then. De Administrando Imperio, a Byzantine diplomatic "guidebook", says Serbs came from "White Serbia" which was neighbor to Francia in 7th century. It is not very reliable source (due to being written centuries after these events, and other reasons), but if true, it is therefore Serbs who split off from Sorbs and migrated south.
But that was 13 centuries ago and not really relevant pertaining to modern peoples.