We have two extremes to avoid here.  Genetics do increase the probability of a language being related, but obviously you can't assume that genetics are going to intuitively carry one's language.
My mother is French and German, but I speak Spanish better than both and she speaks only English.  I'm mostly Irish on dad's side, but neither of us speak Irish Gaelic, though one day I hope I can get the gumption to learn that great language.  Yet, there are genetic commonalities between an Irish and an Indian or a Hausa-speaker and an Hebrew-speaker, and these need to be understood to grasp the big picture of genetic, cultural and linguistic development through the ages.