Complex sentence: In the Netherlands, South Guelderish dialects are subject to the standardized form of Dutch; to which it, being a Dutch dialect, it is very similar, which means it has been relatively uninfluenced.
Split into simpler sentences: In the Netherlands, South Guelderish dialects are subject to the standardized form of Dutch. Since it is a Dutch dialect, it is already very similar to the standard language, so it has been relatively uninfluenced.