Representative theologians and clergymen from both the RCC and the EO have proclaimed each other as "sister churches." The agreements made at Balamand, Lebanon comes to mind, but I'm sure other more recent examples may be found.
Unfortunately, I see several problems. The Latin Church still sees herself exclusively as the Catholic Church. She still refers to other churches (such as the Eastern Catholic Churches) only in relation to the Latin Church, which is equated by many Latin Catholics with the Catholic Church. A communion of churches ecclesiology must needs be better stressed. The Latin Church also, traditionally, sees herself, her Pope, her bishops, the theology they all profess and the liturgy they offer as THE Catholic faith, any difference or deviation from which is an indication that one is not fully Catholic. Yes, there is lip service paid to the equal status of the Eastern rites, but in reality, in practice, Latin Catholics commonly view the Eastern traditions as not as developed (and therefore not as profound or completely true) as the Latin traditions. In order for the Latin Church to regard the Eastern Churches as sister Churches and not as daughter churches, she needs to better appreciate and accept the full legitimacy of Eastern traditions, as well as the Eastern mindset, which baffles those Western Christians who want to know everything in precise details and who see mystery as obfuscation.
The Orthodox Church, on its part, would have to recognize that there exists a legitimate Western Orthodoxy that is different in many respects from Eastern Orthodoxy. The Eastern Orthodox Church would have to allow this theological and liturgical plurality to take place.