On the official SIR website, in the Theology section, there is a paper titled: The Unity and Common Perspective of the Old Calendarist Orthodox Anti-Ecumenists of Greece, Romania, and Bulgaria (October 2, 2006). This statement defines the Sister Churches' basis for unity. At the time the paper was written ROCOR was barreling blindly into the union. Prior to the Laurus Synod, ROCOR was a member of the Sister Churches. And now again, Remnant ROCOR (Agafangel Synod) has returned to her original path and been welcomed back by the Sister Churches. Here is an excerpt from the 2006 statement concerning ROCOR's sad straying from the original path:
e. To this God-pleasing work of the Anti-Ecumenists, this fundamental union, which seeks, by the synodal condemnation of Ecumenism and the restoration of the Old (viz., the Church or Patristic) Calendar, the pacification and reunion of those who are separated, the Russian Orthodox Church Abroad was also, in the past, resolutely tied, particularly under the Most Holy Metropolitan Philaret (1965-1985) and thereafter, both in its Patristic stand against Ecumenism and by its communion with the Old Calendarist Anti-Ecumenists of Greece, Romania, and Bulgaria.http://www.synodinresistance.org/Theology_en/E3a3a009bKoineDelosis.pdf
f. However, in the last few years, we have undergone a great spiritual trial, by verification of the fact that the venerable Russian Orthodox Church Abroad has steadfastly moved towards ecclesiastical union with the Patriarchate of Moscow, and through it with all of the Orthodox Ecumenists, thereby ceasing to constitute a prominent bastion in confronting the syncretistic heresy of Ecumenism; furthermore, this is an indisputable fact, since the Church Abroad has communed wholly openly with the Patriarchate of Serbia (May 2006), which is in clear violation of the Anti-Ecumenical Legacy of the ever-memorable Confessor, Archimandrite Justin Popovich, and which in various ways is already playing a leading role in pro-Papist and ecumenical developments.
Two other good defining papers that answer many questions:
An Ecclesiological Position Paper
The True Meaning Of Canonicity