Ethnicity is a notoriously difficult concept to define.
There is little evidence of territory shaping any identity in any particular way. For example, mountainous regions have produced cultures as varied as Quechua, Karen, and Montenegrin.
Ideas of ethnic unity are very dubious. Theory of innate human difference along ethnic lines is widely discredited as a pseudo-science.
Ideas of kinship struggle to explain nations which contain many different ethnic groups such as the American nation of immigrants or India which contains over 2000 ethnic groups according to some counts.