The language of “separated brethren (brothers and sisters)” appeared in 1964 (Vatican II - Unitatis redintegratio ) and again in 1995 (Et Unum Sint). I believe that it can be found in additional sources both older and newer.
From Pope John Paul II Encyclical “Et Unum Sint” (1995):
36… Dialogue is also a natural instrument for comparing differing points of view and, above all, for examining those disagreements which hinder full communion between Christians. The Decree on Ecumenism dwells in the first place on a description of the attitudes under which doctrinal discussions should take place: “Catholic theologians engaged in ecumenical dialogue, while standing fast by the teaching of the Church and searching together with separated brothers and sisters into the divine mysteries, should act with love for truth, with charity, and with humility”.
Love for the truth is the deepest dimension of any authentic quest for full communion between Christians. Without this love it would be impossible to face the objective theological, cultural, psychological and social difficulties which appear when disagreements are examined. This dimension, which is interior and personal, must be inseparably accompanied by a spirit of charity and humility. There must be charity towards one’s partner in dialogue, and humility with regard to the truth which comes to light and which might require a review of assertions and attitudes.
John Paul II. (1995). Ut Unum Sint. Vatican City: Libreria Editrice Vaticana.
47… Dialogue does not extend exclusively to matters of doctrine but engages the whole person; it is also a dialogue of love. The Council has stated: Catholics must joyfully acknowledge and esteem the truly Christian endowments from our common heritage which are to be found among our separated brothers and sisters. It is right and salutary to recognize the riches of Christ and virtuous works in the lives of others who are bearing witness to Christ, sometimes even to the shedding of their blood.For God is always wonderful in his works and worthy of admiration”.
John Paul II. (1995). Ut Unum Sint. Vatican City: Libreria Editrice Vaticana.
48…The Second Vatican Council made it clear that elements present among other Christians can contribute to the edification of Catholics: “Nor should we forget that whatever is wrought by the grace of the Holy Spirit in the hearts of our separated brothers and sisters can contribute to our own edification. Whatever is truly Christian never conflicts with the genuine interests of the faith; indeed, it can always result in a more ample realization of the very mystery of Christ and the Church”. Ecumenical dialogue, as a true dialogue of salvation, will certainly encourage this process, which has already begun well, to advance towards true and full communion.
John Paul II. (1995). Ut Unum Sint. Vatican City: Libreria Editrice Vaticana.