Sister church should not be confused with Mother church.
Sister churches was a term used in 20th-century ecclesiology to describe ecumenical relations between the Roman Catholic Church and the Eastern Orthodox Churches, and more rarely and unofficially, between the Roman Catholic Church and the Anglican communion. The Catholic Church has since called on theologians to avoid the term, clarifying that "one cannot properly say that the Catholic Church is 'sister' of a particular Church or group of Churches. This is not merely a question of terminology, but above all of respecting a basic truth of the Catholic faith: that of the unicity [uniqueness] of the [Catholic Church]." The term is also currently used among Protestants to refer to different denominations of the same religious tradition.
See also: Christianity in the 12th century. The expression, allegedly in use among the Orthodox since the fifth century among the "patriarchal sister Churches", appeared in written form in two letters of the Metropolitan Nicetas of Nicomedia (1136) and the Patriarch John Camaterus (in office from 1198 to 1206), in which they protested that Rome, by presenting herself as mother and teacher, would annul their authority. In their view, Rome was only the first among sister churches of equal dignity, see first among equals. According to this idea of Pentarchy, there are five Patriarchs at the head of the Church, with the Church of Rome having the first place of honor among these patriarchal sister churches. According to the Congregation for the Doctrine of the Faith, however, no Roman pontiff ever recognised this Orthodox equalization of the sees or accepted that only a primacy of honor be accorded to the See of Rome.
In modern times, the expression "sister Churches" first appeared in John XXIII's letters to the Orthodox Patriarch of Constantinople, Athenagoras I. In his letters, the pope expressed the hope of seeing the unity between the sister churches re-established in the near future. Later the term appeared in a "Joint Declaration" between Pope Paul VI and Patriarch Athenagoras in 1965, representing their respective sees, Rome and Constantinople, respectively.
The Second Vatican Council adopted the expression "sister Churches" to describe the relationship between particular Churches: "in the East there flourish many particular local Churches; among them the patriarchal Churches hold first place, and of these, many glory in taking their origins from the apostles themselves. Therefore, there prevailed and still prevails among Eastern Christians an eager desire to perpetuate in a communion of faith and charity those family ties which ought to exist between local Churches, as between sisters."
The first papal document in which the term "sister churches" is applied to the Churches is the apostolic brief, "Anno ineunte," of Paul VI to the Patriarch Athenagoras I. After having indicated his willingness to do everything possible to "re-establish full communion between the Church of the West and that of the East", the Pope asked: "Since this mystery of divine love is at work in every local Church, is not this the reason for the traditional expression 'sister Churches', which the Churches of various places used for one another?"
More recently, John Paul II often used the term, especially in three principal documents: