You need to start by speaking to the IP owner of the Simpsons.
They won't even acknowledge you though unless you have a proven track record of making good games and/or tons of money and a big publisher behind you to negotiate with their legal team.
Do you have any of these things before you start?
DON'T, I REPEAT, DONT write any code for the game until you have permission as you'll just be wasting your time...
Good luck!
Thanks for your help, it was just a little project i wanted to undertake. I guess its not really possible then and i should give up.
Thanks,
Ben
Have you considered creating a spiritual successor?
Create your own dysfunctional family of five, and the same GTA style mechanics and missions. Before you know it you'll be creating something original with its own ideas. Remember that saints row started off as a cheap GTA knock off and it evolved into something completely different and just as good by the second and third episodes. Bioshock was spiritual successor to system shock 2, but they are very different games, the newer of the two having most of the RPG elements stripped back, and a more moralistic plot...
I'm sure you have lots of ideas in your head that might result in a good game... Just don't outright copy Simpsons hit and run