But you must accept them as brethren.
Catechism of the Catholic Church:
838 “The Church knows that she is joined in many ways to the baptized who are honored by the name of Christian, but do not profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter.”322 Those “who believe in Christ and have been properly baptized are put in a certain, although imperfect, communion with the Catholic Church.”323 With the Orthodox Churches, this communion is so profound “that it lacks little to attain the fullness that would permit a common celebration of the Lord’s Eucharist.”324
Since the Sacraments were all instituted by Christ, they belong to the Church He founded. One Lord, one Faith, one baptism. Those baptized Christians who reject His One, Holy, Catholic and Apostolic Church are the ones who are in effect rejecting Catholics as "brethren".
With the Orthodox Churches, this communion is so profound “that it lacks little to attain the fullness that would permit a common celebration of the Lord’s Eucharist.”324
But it does lack the fullness (which the catechism also states). The existence of the gulf rather than its degree is the material point, ecumaniacal delusions not withstanding.