Complex sentence: Native Sikkimese are the indigenous peoples of Sikkim, including Lepchas and Bhutias, in contrast to Nepali people and Tibetan people in Sikkim.
Split into simpler sentences: Native Sikkimese are the indigenous peoples of Sikkim, including Lepchas and Bhutias. That is used in contrast of Immigrant Sikkimese, a term referring to Nepali people and Tibetan people in Sikkim.