The problem is that Lois, Meg, and Chris are too similar to the characters they are ripping off so there's only so much you can do before they start doing out-of-character things. In the early days you could have fixed that, but now it's too late. Lois is a loud-mouth version of Marge Simpson. What else is there to do? Meg is a teenage Lisa Simpson except she isn't smart. Again, what else can you do with the character? And, Chris is basically Ralph Wiggium. Again...not much you can do with the character.
Stewie is the only original character, so there's a lot that you can do with him before you reach the "out of character" zone. Peter is Homer Simpson, but he's an extremely over-the-top version so you can almost do anything like him running out of the house and into a helicopter with his face on it. Something you'd never see Homer Simpson do. And, Brian is your typical straight man so he works great with either character.
