Played it a lot before, right now just waiting for Cataclysm (and frankly The Old Republic), just raiding with the guild as I don't wanna bail on them (and it's still a bit of fun with a couple of good online friends).
WotLK kinda sucks, but if you just started I doubt you'll notice it, you'll have plenty of fun just hanging out with your sis and leveling, even if you reach 80 and get a bit bored (which again, is unlikely since you're new and Cata's not that far off), you'll still have plenty of classes to level out and mess around with. I'd suggest trying out both Alliance and Horde btw, the leveling is quite different and makes for a more enjoyable experience the second/third/Nth time around.
I prefer Alliance for the races (though not by far), but like the Tauren/female Blood Elves, so both sides are cool really.