Complex sentence: Siberian Tatars refers to the indigenous Turkic population of the forests and steppes of Western Siberia stretching from the Ural Mountains to the river Ob, a sub-group of the Tatars, sometimes considered a separate ethnic group.
Split into simpler sentences: Siberian Tatars refers to the indigenous Turkic population of the forests and steppes of Western Siberia stretching from the Ural Mountains to the river Ob. Therefore this sub-group of the Tatars, sometimes considered a separate ethnic group.