Watching the marathon, I seem to recall Chip Fields playing Shirley's sister in one episode and then playing I think Rerun's sister in another episode.

I think you are right about that. I am positive she played Rerun's sister.

Watching the marathon, I seem to recall Chip Fields playing Shirley's sister in one episode and then playing I think Rerun's sister in another episode.
yeah, she played Shirley's pregnant sister, who was abandoned by her goofy boyfriend. That actor played the nerd in "Thelma's apartment" on Good Times. Then, Chip played Rerun's sister, the nurse.