I generally have my tank be my main. If you brother is going to be tank I might consider trying your healers together with a Vetra and you DPS together with another Vetra. With that group you won't have access to tracking which can be very nice. Also not sure on evacs with the warlocks (the warden has it but they are usually concentrating on heals when an evac is needed).
My main group set up in Paladin, Warden, Mystic, Swashbuckler, Conjurer, and Wizard. Not a lot of planning went into my classes as I started out 3 boxing and added alts as time went on.
Perhaps warden, warlock x2, defiler, and troubador. With the tank played outside of multiboxing. As you said that would be a decent group.