I have discovered Sissy's true identity. He really is Navin R. Johnson. Below are some quotes from Navin:
Navin R. Johnson: Huh? I am *not* a bum. I'm a jerk. I once had wealth, power, and the love of a beautiful woman. Now I only have two things: my friends, and... uh... my thermos. Huh? My story? Okay. It was never easy for me. I was born a poor black child. I remember the days, sittin' on the porch with my family, singin' and dancin' down in Mississippi...
Mother: Navin, it's your birthday, and it's time you knew. You're not our natural-born child.
Navin R. Johnson: I'm not? You mean I'm gonna STAY this color?
Navin R. Johnson: Don't trust whitey. Lord loves a working man; don't trust whitey.