See 2 John 1:1. It is most likely that John is referring to another Christian woman and her household who were sending their greetings. Calvin summarizes 2 John 1:12-13 saying:
The apostle concludes this letter, first, by adjourning many things until he meets them personally (2 John 1:12). He supposes that some things are better spoken than written. The use of pen and ink may be a mercy and a pleasure; but personal contact may be more so. The apostle was not yet too old for travel, nor consequently for traveling service. The communion of saints should be maintained by all methods, and their communion should produce mutual joy. Excellent ministers may have their joy advanced by their Christian friends. "That you and I may be mutually encouraged by each others faith" (Romans 1:12).
Second, he presents the greetings of some near relatives of this lady. Grace was abundant toward this family; here are two chosen sisters, and probably their chosen children.
Once again, in this book we have seen magnificent scriptural truths that are consistent with the great cry of the Reformation:
Sola Fide (by faith alone)
Sola Scriptura (by Scripture alone)
Solus Christus (through Christ alone)
Sola Gratia (by grace alone)
Soli Deo Gloria (glory to God alone)
chosen sister. John uses the same metaphor from v. 1. This is the only place where this kind of description is used in the NT, but the family language most likely refers to the church (see note on v. 1). Here, two sister churches are sharing intimate family greetings.