Complex sentence: Mistresses of this kind mostly come from lower social class than the man and they rank in status below the wife wed by the church, the arrangement can be called a semi-polygamy, but children born to such unions are socially the equals of their brothers.
Split into simpler sentences: Mistresses of this kind mostly come from a lower social class than the man and they rank in status below the wife wed by the church. The arrangement can be called a semi-polygamy, but children born to such unions are socially the equals of their brothers.