It follows two sisters— one is devoted to her faith, the other breaks the community's strict rules.
What would we class this type of clause ? (one is this, the other is this)
What part is the main clause ?
It is a "comma splice", but a fairly harmless one. Each part is a complete sentence, but the two sentences are linked. By splicing the two sentences with a comma the author indicates that they should be read and understood together. You could punctuate differently using a semicolon to avoid the splice:
It follows two sisters: One is devoted to her faith; the other breaks the community's strict rules.
Comma splices are generally considered to be poor punctuation and should be avoided, but this one is fairly harmless.