"Sister" is a symbol of a figurative "Gathering" of spiritually related like-minded people in the understanding and fulfillment of God's Will, His Commandments, a community of people united by a common true Faith, a godly way of thinking and way of life. 

Negative Meaning.

"Sister" is also religious, political and other communities of people who are related by a vicious, ungodly way of thinking and living, who reject, pervert God's Covenants, who turn to serving false teachings, "Idols", idols, who create only the appearance of doing the Will of God, and therefore falsely understand and spread their distorted Word of God, false understanding of the sayings of the Prophets, Christ and the Apostles.  

Translated with DeepL.com (free version)