That's really a shame. May she rest in peace.
Also an interesting bit of trivia: She did not appear in Back To the Future Part II because she was pregnant at the time of filming. Robert Zemeckis and Bob Gale originally intended for both of Marty's siblings to be in the "alternate" 1985. The character of Marty's brother, Dave, did actually have a part filmed for the movie where he became a bum; however, because Wendie Jo couldn't be in the film, they didn't want to only show one sibling, and therefore decided to cut the scene with Dave altogether.