Meg isn't a INFJ. She's not deep enough, nor is she an NF to begin with.
NFs seek validity over anything, and that is Meg's primary goal. I'd say she's an INFP, just not a very deep one because she's young yet. (intuitive =/= smart and sensor =/= dumb!)
Brian is INFJ he's cultured, complex, and he's the one Peter goes to for advice. He's tough but has a humanitarian bent. haha. wow...
No way, Brian is most certainly a thinker, and much more Ne oriented then Ni, while his dominant function tends to be Ti.
He's Ne oriented over Ni because he's always open to new possibility, and is almost making small speeches saying, "Well, maybe our situation is actually this way, which is comforting and better explains our behavior." He also just happens to be humanitarian, which isn't a trait exclusive to NFs (I'm INTP and I'm extremely humanitarian, a pacifist, am very moved by compassion toward other people, and have an ethics that holds loving-kindness as one of its 3 intrinsic values...I just tend to make decisions based on logic and rules derived from these values, and enjoy activities that invoke my logic).
So this puts him as an INTP or ENTP. An ENTP is a type that has a wild streak. They often do completely inappropriate things just to see the reaction of others. They're very experimental with life and see it as one big science project. INTPs are more apt to seek to explain that science project, and not just experiment in it. They want to find the best explanation through a sea of possibilities. They can be experimental with life when they force themselves to be (or others force them to be), but for the most part are reserved and don't draw attention to themselves.
I'd say Brian is an INTP. He fits the description awfully well. He's reserved, open to new experience but doesn't demand it often, and seeks clarity to the show's dilemmas, often starting a monologue that presents a new way of looking at what has gone wrong.