This was supposed to be the GameCube's equivalent of Super Mario 64, but obviously things didn't pan out that way. Despite mixed reception, the game became commercially successful, found a devoted fanbase, and apparently warrants a sequel on 3DS (which isn't out yet as of this writing).
Mario and Luigi Mario was saved by Luigi in 2012's Luigi's Mansion, Dark Moon and in it's Prequel. They've been known as brothers since Luigi came out, and hopefully will stay that way. Brothers forever.
It's Melody playing the piano! Jarvis is the one in the jar, the one sleeping is sue pea (she had an upside down room) The one coming from the ceiling way in the back is Bogmire(he was in a graveyard) The two twins fighting are Henry and Orville, the butler, sadly I do not know(I always just called him "the butler"), but he would walk around invisible, his flames alight, until you followed him into the billiards room, and the dancing couple is the Floating Whirlindas, or something like that.