The theme of II John is that of false teachers. The nineteenth of the Epistles was written from Ephesus sometime around 93 AD by the apostle John; the last surviving of the 12 disciples of Jesus. They are still causing problems for the church, wandering around trying to convert people to follow them. John fears that these itinerant preachers might take advantage of the good hearted Christians, and so he warns them not to listen to those who do not speak the truth about Jesus.
"The children of thy elect sister greet thee. Amen."
The children of one's sister would be one's nephews and nieces, but it seems that John is referring to the congregation of a sister church