Despite being major characters in the "Back to the Future" trilogy, Marty's parents George and Lorraine McFly never appeared in the series.
While, out of the entire cast of all three movies, only Mary Steenburgen (Clara) and Tom Wilson (Biff) reprise their roles in the animated portion of the show, Christopher Lloyd appeared as Doc Brown in live-action bumpers (the parts between the show and the commercials) and in a live-action segment at the end of each episode with Bill Nye, teaching kids about science.
In Brothers (1991), Marty mentions Dean Strickland. Mr. Strickland from the first two movies is now Dean of Hill Valley college.
In Hill Valley Brown-Out (1992), one of the films playing at the Hill Valley Movie Theater is "Back to the Future (1985)."
There were plans for a character named Polly Clayton to appear in the series. Polly, aged nine, would presumably have been a relation of Clara's, but the character was dropped before the series aired.