steve-b:
“It must always be clear, when the expression
sister Churches is used in this proper sense, that the one, holy, catholic and apostolic Universal Church is not sister but
mother of all the particular Churches.
[8]”
Yes, the Catholic Church as a whole but at the local level the expression
sister Churches may be used. Here is the sentence just before the one of which you quoted, “10. In fact, in the proper sense,
sister Churches are
exclusively particular Churches (or groupings of particular Churches; for example, the Patriarchates or Metropolitan provinces) among themselves.”
There are two examples of this in the document. First, “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.»” and, " 7. The first papal document in which the term
sisters 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? For centuries
our Churches (Rome and Constantinople) lived in this way like
sisters, celebrating together the ecumenical councils which defended the deposit of faith against all corruption. Now, after a long period of division and mutual misunderstanding, the Lord, in spite of the obstacles which arose between us in the past, gives us the possibility of
rediscovering ourselves as sister Churches.»
So, the Pope, as the bishop of the diocese of Rome and the Ecumenical Patriarch, as the bishop of Constantinople, can refer to each other as heading
sister Church.
ZP