Two issues here, first the “Mother Church”. Even the Pope has said that the Orthodox Churches are true Churches, he just says they’re “defective”, because they are not in communion with Rome. But since the are Churches (as opposed not being a Church, as protestant are, because they don’t have bishops) then those where once a part of one of the Orthodox Churches, but became one of the Eastern Catholic Churches, why wouldn’t the Orthodox Church from which they came be called their mother church? They didn’t come from the Latin Church.
The other issue is why say one is in communion with Rome. It is very important that all true Churches be in communion with each other, because all bishops should be in communion with each other, because the episcopacy IS one. (Their are reasons that can justify one church braking communion with another, but this is always seen as temporary with a view to restoring communion as soon as possible). The Church teaches that in order to show that the Church and the episcopacy is one our Lord first gave the Keys of the Kingdom to one person, namely St Peter. This “one” is continued today in the man who is 1st hierarch of the whole universal Church. This is of course the Pope, the bishop of Rome. (Just how it fell upon Rome, or if Rome still holds it is a disputed matter today between RC & EO, but both believe that the unity of the whole Church comes from the one who is the 1st ranking hierarch). So, seeing that being in communion with all Churches is important, it becomes even more important that a Church be in communion with the one Church that has the primacy, since Catholic unity comes from the 1st hierarch of this Church - which is the Pope. Most believe that the only thing that can justify not being in communion with the Church that holds the 1st place is if that Church has fallen into heresy. The Eastern Orthodox officially say that Rome has fallen into heresy, so the 1st hierarch for the EO falls upon the one who was in the 2nd place before, and is now considered the new 1st hierarch, and that would be the bishop of Constantinople. - And, if I may add my opinion, I do not personally believe that Rome is in heresy, so I am struggling with this matter; but I (for now anyway) have concluded that there are more reasons that can justify not being in communion with Rome other than heresy only.
Peace!