The Wii is not designed with the hardcore gamer in mind, but it's definitely a good family system. I have both a Wii and an XBox 360. I play the 360 a lot more than the Wii, but my fiancée absolutely loves the wii. She got really into Mario Galaxy, and I downloaded the Wii Ware game Tetris party, which she has been addicted to. It's nice because you can play against other people online in Tetris, and she has really enjoyed that. World of Goo (another Wii Ware game) has been fun, I just got that a few days ago.
We rarely play Wii Sports anymore (i like my games to have more depth), but WiiFit should be arriving tomorrow, which should entertain us. I've also gotten a decent amount of gameplay out of Mario Kart and Super Smash Bros, although the usage of those fell off after a few weeks... They're just a lot more fun with a group of people, and I'm not in college anymore with multiple friends readily available.
For your situation, i'd definitely recommend the Wii. Sounds like you have a built in party, with five people in your household. You should get a lot of value out of a lot of the multi-player games.
Side note regarding the Wii Fit... Technology is friggin' amazing. I have been watching for it to be in stock on Amazon for awhile, not wanting to pay the $150 that people are asking on the secondary market. I noticed that Amazon offers text alerts for when they get an item in stock, so I signed up for that. Digging in deeper, I saw that you can configure your text settings so that when they text message you, you can just respond to it and tell them to buy it, without having to actually be anywhere near a computer. So that's basically what happened on Tuesday!