World

"They are not of the world, just as I am not of the world." John 17:16

To be a Christian is to belong to the Kingdom of God and deny the kingdom of this world. You can't belong to both kingdoms simunteounesly, just like you can't serve two lords.

Brothers & Sisters

"Behold, how good and how pleasant it is for brethren to dwell together in unity!" Psalms 133:1

The unity of faith depends in the unity of the Spirit of God. If there's no unity among brothers and sisters, it's because they don't have the same Spirit. In this case, it's better to avoid such kind of 'brothers and sisters'.