Specifically: "The character must be of a minimum level equal to (monster level -5), or (lowest attacker level -5), whichever is lower, to receive passive split EXP."
Since in your party you are over 5 levels away from both the monsters and your brother, you only get exp from mobs you hit. Once you level up to 125 you should begin to receive the "passive split" exp from his kills of level 130 Moon Bunnies.
The Cygnus you need to defeat for Cygnus Knight 4th job advancement is a special quest version. No-one can go in there with you, but you shouldn't have any trouble with it.
