Hey, I'd suggest doing the mage. I currently play a beastlord with my brother's mage, and it's a lot quicker. Necros rely more on DoTs to get their DPS in, while their pet functions as a speed bump. Mages have uber pets (plus they can create pet toys for both pets, beastlord and mage), do burst DPS and give damage shields to help the mob die faster. But necros have FD, so you wouldn't die as often. If your sister plans on tanking with her beastlord, the mage probably is a better bet, since beastlords can't tank so well until after defensive AAs and lots of AC gear. Mage would be able to kill the mob before it kills the beastlord. Hopefully.

Cliffnotes: Necro = long fight, relatively safe, can take on red mobs via kiting. Mage = short fights, utility benefical to pets, probably can't take on red mobs. That is, until you get into AAs, which is a completely different story.

Mercs: My advice would be to get one healer merc and a DPS merc (rogue if beastlord tanks and holds aggro, wizard if the pets tank) if you and your sister play together. If you play separately, go with a healer merc on both toons, so both can solo :)