Although Michael was the ultimate and sole boss as he made clear to Frankie, I think with Michael's move to Nevada and interest in the more lucrative and white-collar businesses/crimes, Frankie is effectively the boss of the NY Corleone group.

I think everyone who needs to know , knows that Frankie answers to Michael. I thought of Cicci either as a "special" or a capo by that point. As one of the people who took care of some very sensitive tasks during the Corleone "restoration" as it were I think he would have risen in authority.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.