I don't know American Dad but Family Guy I would say:
Peter- Pisces (He is very imaginative he tends to like his freedom and he doesn't care what people think; Can be lazy and doesn't come back to reality on important matters)
Lois- Libra (Thought of as being beautiful she can be peaceful one of the bunch; sometimes she can be flighty/flirty in some episodes)
Brian- Virgo (He is very helpful always there when you need him he is observant, precise, intelligent; he can sometimes become very fussy/cold)
Stewie- Gemini (He is intelligent, clever and witty and has a huge imagination; can be really mean and impulsive behavior)
Meg- Taurus (No matter what Meg is put through she is very loyal and generous to her family; she can be stubborn and sometimes materialistic but to prove points)
Chris- Cancer (Chris is very nice and responsive to everything he is always friendly; he is sort of clingy and overemotional sometimes)
Cleveland- Sagittarius (He is very independent and a great person to go to talk to he can let a bad situation go when need be; sometimes unemotional and letting too much go when should be fixed)
Joe- Aries (Joe is courageous and helps out people he cares for; he can be a little impulsive and short tempered with certain situations)
Quagmire- Scorpio (Well Quagmire is certainly passionate and resourceful lol; he can be a little obsessive and manipulative)