It is generally assumed that semitic languages like Hebrew and Arabic are similar because the speakers of those languages are ethnically related and share a common ancestry. In other words, A Hebrew speaker is ethnically more closely related to the Arab than he is to the speaker of English or French. And so it goes for the Indo-european languages, it is assumed that they are similar because the people share some kind of common ancestry.
My question is, is this rule set in stone? Might there an exception to this rule? Can anyone give an example of two languages that are very similar to each other, yet their DNA shows that the people are ethnically unrelated to each other? I can easily imagine a scenario of an Asian people (speaking an Asiatic language) wandering into the European continent and settling there, eventually they assimilate with the Europeans (yet still retain their distinct identity and culture) and learn the Indo-european language and end up adopting it. Eventually their languages may end up sounding very similar, yet the speakers are ethnically unrelated. I would appreciate if anyone can share any examples or new research on this.