Sister-in-laws' because it is plural not singular. When a noun ends in an 's' whether it is plural or that is just the way it is spelled, you add the apostrophe at the end of the word and no "s".
Karl Marx who believed that religion was a source of social control through which the lower classes could be suppressed. And he wished to end this status quo and source of social inequalities so the proletariat could rise from oppression and claim rewards in this life.
