Who Understands Stewie

Does everyone understand Stewie?

According to Seth MacFarlane, himself, everyone IS able to understand Stewie full well.

Why can only Brian and Chris understand Stewie?

This is akin to people making baby-talk with a toddler – they do engage him in conversation, but they are not as elaborate as Stewie interprets them. That’s why Brian, a dog, talks. He sees Brian as another member of the household, not able to understand that different species cannot communicate with one another.